Moscow’s announcement on Sunday, February 27 that it had raised the readiness status of its nuclear forces, sent a cold chill down the spines of anyone in the West listening to the news. Kremlin Spokesman Mr Peskov stated, while announcing the change, that it was triggered by an (unspecified) threat issued by the UK’s new Foreign Secretary Liz Truss the day before.

The exact nature of the threat has not yet come to light and the reference may be nothing more than an excuse, though it may also have substance, since Ms Truss only came to her office four months ago, after a political career spectacularly clear of foreign or defence experience (her last job was Minister for Women and Equalities).

A more likely explanation for Moscow’s step is exactly to freeze those spines, and to give the West pause for thought. Taking a leaf out of Kim Jong Un’s playbook, if one makes oneself appear irrational, or even unhinged, one’s opponents tend to step more carefully around one’s interests.

Whatever the reason, does a raised alert status mean anything in practice?

Three generations of nuclear weapon design and nuclear strategy have resulted in a simple arrangement of nuclear forces among the world’s five major nuclear-armed powers.

The largest forces, those of the US, Russia and China, are divided between missiles based on land in deep protected bunkers, and those based at sea in large nuclear-powered submarines. Missiles based in bunkers are by their nature first-strike weapons, because their locations are known to each side. This means that in a nuclear exchange they will be at the top of the target list, and therefore unlikely to be available for a second or subsequent strike.

The calculus of nuclear warfighting, though, rarely reaches 100% of anything. A nuclear missile bunker is built deep and hard, designed to withstand anything less than a direct hit from a large (200-300 kilotonne) warhead. Since even the most accurate missiles contain a target error (the “Circular Error Probable” or CEP) and the most reliable missiles can misfire or mis-fly, a first strike on land-based bunkers is highly unlikely to kill all an opponent’s land-based missiles. Hit probability is reduced further if one’s opponent has an effective anti-missile system. It is possible to increase the kill probability by making the warhead larger, but that means making one’s own missiles bigger, increasing cost of the missile and its bunker, and so reducing the number of missiles available. 300 kilotonnes is about the practical limit for land-based missiles.

A strike on missile silos is named a “counter force” strike in the trade, compared with strikes on cities and industrial zones, which are named counter-value.

The second-rank nuclear powers – UK and France – have had no land-based missiles since the mid-1960s.

To balance the high probability that most of one’s land-based missiles will be destroyed in a first strike, all five leading powers developed sea-based missiles to be carried in large nuclear-powered submarines. The advantage of sending one’s missiles to sea (at much higher cost) is that a dived submarine can move about the ocean at will and undetected, rendering its missiles effectively invulnerable to a first strike. Submarine Launched Ballistic Missiles (SLBMs) therefore are thought of as a retaliation weapon. With this two-legged strategy an operator of SLBMs can guarantee a massively destructive retaliatory strike on an aggressor. This two-part structure has proved to be remarkably stable – none of the three larger powers has initiated a nuclear war.

The minor nuclear powers – Israel, India and Pakistan – have neither land silos nor nuclear submarines. These rely on lesser delivery systems – medium range rockets or weapons delivered by aircraft. The nuclear minnow – North Korea – is a special case that is not relevant to this article.

Both American and Russian land-based missiles must fly approximately 10,000 km from silo to target. That is the ground distance. In practice the missiles fly further, taking a parabolic route well above the earth’s atmosphere. That makes flight time from launch to detonation about 25 minutes. UK and French targets are half that distance from Russian silos.

Both sides conduct permanent (classified) satellite surveillance of each other’s land mass to detect the burst of intense heat caused by a rocket launch, but launch warning alone is not enough to justify a counter-strike, so powerful long-range radar systems back it up. By the time an incoming missile has climbed high enough to be detected by radar it has only about fifteen minutes to fly (four or five for the UK, hence the mythic 4-minute warning).

Therefore, on the assumption that the first missiles seen are counter-force and aimed at one’s own land-based silos, one’s own missiles must be ready to launch and get well clear of the incoming warheads to avoid being damaged within less than 15 minutes of an order to launch.

Launch requires a combination of physical tasks and command processes. The physical tasks are to run up the missile’s guidance gyros, to programme its target from a range of pre-planned portfolios, and to open the silo door – a massive concrete cap. A checklist of preparations is then run through by the team of two launch officers. From receipt of command instruction to “birds away” takes about seven minutes.

Before the silo officers initiate launch their command authorities must choose the target package. This includes allocating their stock of 450 (US) and 312 (Russia) missiles between counter-force and counter-value, or retention for later strikes. Once a command choice has been made, the relevant orders must be transmitted to the launch officers for verification and execution. It is obvious that the command process takes time, which is why US equipment to authorise firing orders is carried by an officer in a large briefcase (called the nuclear football) within a few seconds’ reach of the President at all times.

Once the command to fire has been authorised it must find its way down the command chain through secure networks, and needs to be verified by each team of silo launch officers. It also needs to be supplied to the permanent airborne command team who fly 24/7 in case a first strike destroys silo command stations (remote from actual missile silos).

Everything described here has applied at every moment of every peaceful day since around 1965, when the challenge of nuclear command was addressed in detail (what happened between 1945 and 1965 truly freezes the blood, but is now history so not for this article). It may be seen that all nuclear forces therefore live permanently in a state of high alert. The concept of increasing that alert status therefore has little practical meaning.

Launch notice for submarine-launched ballistic missiles is a little more relaxed, but not by much. Hidden deep in large oceans submarines are in practice not vulnerable to a counter-force attack. Their missiles are therefore intrinsically designed to inflict counter-value retaliation for an attack – the “assured” part of Mutually Assured Destruction.

SLBM command and control systems are similar to land-based systems, with the difference that it is harder to communicate with a deep dived submarine than with a silo in North Dakota or Siberia. At depth the only radio signals that can penetrate are Ultra-Low-Frequency ones. ULF implies a long wavelength, and a long wavelength means a low bit-rate, so ULF radio transmissions can only effectively be used as bell-ringers, to bring the submarine up to a depth to receive normal radio signals.

These steps, the transmission of a bell-ringing alert and a return to periscope depth, add several minutes to the launch process.

At periscope depth the time taken to issue and verify a launch command are similar, and the missile preparation is also similar: internal guidance systems are run up and a target package is programmed in.

Some additional steps add delay. First, to be usefully accurate as a counter-force weapon a ballistic missile must be told precisely where it is being launched from. A yard of error at launch is a yard of error on arrival. On land that’s easy – the position of each silo is permanent and known to within an inch. At sea the submarine must provide the missile with an accurate launch position. If GPS systems are un-damaged that is easy but takes a little time, but if GPS has been damaged or interrupted the submarine must use its own internal inertial navigation system to programme each missile’s flight path – more time.

The second step takes a little longer. Missiles are fired from a dived position, but the boat must in effect come to a stop and “hover” at periscope depth. That hover is a challenge – 18,000 tonnes of boat has to balance perfectly in a moving ocean with her hull only thirty feet below waves which might themselves be thirty feet in height. The balance point is achieved by an automatic system, but takes a few more minutes to achieve.

Next the missile and its tube must be pressurised with Nitrogen gas. When the missile is fired, through ten metres of seawater, the pressurised nitrogen evacuates the missile body, preventing seawater from flowing in and damaging delicate electronic systems.

Finally, the missile is launched. As 60 tonnes of missile leaves the boat 60 tonnes of balancing water flows in to the launch tube, upsetting the hover and demanding more time before a new balance can be achieved before the next missile is fired. And so on.

With the same 25-minute warning time ballistic missile submarines (called Boomers by the US Navy and Bombers by the Royal Navy) therefore live at a permanent 15 minutes’ readiness to fire. There are no practical steps available to raise that readiness state in a deployed boat.

With on-duty strategic weapons, raising of the readiness state has no material meaning.

The same is less true of weapons which are not on-duty. All five of operators of SLBMs maintain at least one boat at sea ready to fire at all times – Continuous At-Sea Deterrence, or CASD. UK CASD began in 1961, and over the sixty years since it has become evident that in order to have one boat at sea at all times a navy must operate four boats.

While one of the four is at sea at 15 minutes’ notice to fire (generally for patrols of 90 days), the second boat is in or close to her base being maintained, repaired, stored and worked up. The third is in light maintenance and upgrade, or is working up after coming out of deep maintenance, or working down before entering deep maintenance, and is available at some months’ notice as a reserve, and the fourth is in dry dock in deep maintenance.

France and the UK, with four bombers each, can support one at sea at all times. The US Navy, with 14 bombers (or boomers, as they would say) can support three or four. Russia has five new Borei class boats, capable of supporting one at sea at all times, plus six 40-year-old Delta IVs which are of marginal use.

Allowing for boat age Russia is probably capable of supporting two boats at sea at all times, and one additional boat when pressed. It is here that Mr Putin’s remarks may have some practical meaning, in the rushing of another Delta, or even two, to sea to join her two sisters. An additional Russian ballistic missile boat adds up to 96 additional warheads to Russia’s 15-minute arsenal of 1,200 warheads – a fairly marginal addition. When we place that addition into a possible counter-value target package it brings an additional fifty US cities into the firing line (assuming two warheads per city, to allow for misfires), but these are cities like Boulder Colorado, with an average population of 100,000. That is not to say Boulder Colorado does not matter to Boulderians, but it is to say that by the time Russia has hit the 250 larger cities than Boulder, the US has effectively ceases to exist anyway. Surging boats to sea is therefore fundamentally symbolic.

It is also too slow to matter within the context of a conflict which is, one way or another, likely to end within a week or two. A quicker addition is readily available for all SLBM powers. When boats come in for maintenance and re-supply they typically unload their missiles to shore-side storage for the 90 days that they are alongside. A nuclear power can increase the number of missiles available at short notice by keeping them aboard a boat while she is alongside between patrols.

That brings a cost in wear and tear on boat crews who must remain on duty rather than heading home for leave or training, but it reduces the lead time to fire those missiles from months to less than a single day – the time needed to get the boat underwater from where she can fire. In round terms each SLBM power can double the number of on-call SLBMs more or less overnight.

Discussion of readiness states does beg the question of “how much destructive power is enough”? Does increasing readiness by increasing availability inherently destabilise the nuclear warfighting calculus? Perhaps. Throughout the history of nuclear warfighting the two main proponents have always had the option of carrying out a pre-emptive disabling strike

In the spring of 1948 the US Joint Chiefs developed a plan called HALFMOON, which anticipated dropping 133 bombs from long-range bombers on 70 Soviet cities as a pre-emptive strike to stop a Soviet invasion of Europe in its tracks. The bombs in HALFMOON were 20 kilotonne fission bombs, tiddlers compared to a 300 kilotonne Minuteman warhead. HALFMOON looks to a modern observer as psychotic, but it was written shortly after a time when a major air raid on a German or Japanese city could deliver eight kilotonnes of high explosive in a single night.

After Russia developed fission bomb technology in 1949 the US abandoned pre-emptive strike, until it gained a lead in more powerful fusion bomb production (also called thermonuclear or hydrogen bombs). As the US lead grew, plans for a pre-emptive strike found a new life from around 1957 until after the Cuban missile crisis (in the late ‘50s the Joint Chiefs even tried to persuade President Eisenhower to launch a pre-emptive hydrogen bomb strike). Even today the US has formally retained the right to use a first strike.

No matter how successful your first counter-force strike may be in destroying your enemy’s missiles before they can be launched, today’s nuclear force structure leaves his submarine-based missiles unharmed. The hundreds of warheads carried on a daily basis by each side’s boats are ample to deliver a killing counter-value retaliation. Raising the readiness of a few dozen more submarine-launched missiles does not change the calculus much, or at all.

The picture becomes more stressful when we move from the strategic to the tactical arena.

Russia has a stockpile of smaller nuclear weapons for use on the battlefield, at sea or in the air, generally categorised as tactical nuclear weapons or theatre weapons. The borderline between tactical and strategic is defined more by delivery vehicle than by weapon size or yield. Using implosion designs a viable nuclear weapon can be built small enough to sit inside a 152mm shell, though sources suggest that Russia has not actually manufactured one that small, making Russia’s smallest tactical weapon a 200mm calibre shell. Russia has also built nuclear mines for land and sea use, small nuclear warheads for short- and medium-range missiles, and nuclear-armed anti-ship and anti-submarine torpedoes and depth charges. Russia’s air force is capable of lifting and launching nuclear-armed cruise missiles, and of dropping free-fall nuclear-armed bombs (the US has a couple of hundred of these too).

The precise number of such weapons in Russian service is not known, but is estimated to be in the low thousands of warheads, above 1,000 but below 2,000 (one source thinks 500). Russian nuclear doctrine is ambiguous. On one hand Russia has publicly forsaken first use, but on the other it has a clear doctrine of first use of weapons in some warfighting scenarios. It is known that Russian forces regularly practice for the use of nuclear weapons in large-scale war-games, and that Russian tactical doctrine sees battlefield nuclear weapons as simply “larger bangs” rather than as a qualitative escalator.

Tactical weapons are intended for use against large military targets on the battlefield. These include enemy airfields and logistics concentrations and large enemy formations. Since targets will often be uncomfortably close to Russia’s own forces tactical weapons are built with smaller yields, down to one or two kilotonnes. A useful yardstick for blast and size is the Beirut fertiliser explosion yielded approximately 1 kilotonne, and had a killing radius of a kilometre or so.

Russia’s stocks of tactical nuclear weapons are stored at twelve sites, from where they may be forward-deployed to maintenance bases. Each base is associated with a military formation authorised in principle to use nuclear weapons, and of course these formations include naval forces and air forces as well as ground forces. In addition, some tactical nuclear weapons are dedicated to ballistic missile defences around Moscow.

When Mr Putin talks about increasing the readiness state of Russia’s nuclear weapons it is most likely that he referring to the physical distribution of tactical weapons from their storage sites to their maintenance sites, and that this probably applies only to the land-based component of Russia’s stocks. As such, an increase in readiness does not in itself indicate a serious step towards use of nuclear weapons, and at the same time does little to increase the risk of an accidental or confused use of a battlefield nuclear weapon.

A much more alarming step would be the issue of nuclear weapons to actual combat commands in or close to the combat zone in Ukraine. Normally one would declare that to be so unlikely as to be impossible, but given Moscow’s extraordinary decision to invade Ukraine in the first place it is no longer unthinkable.

While the direct effects on Western populations of a tactical nuclear weapon are nil, the political consequences of Russia’s use of a nuclear weapon in Ukraine would be immense, and are certainly incalculable in advance. States which are currently on the sidelines of the Ukraine war may finally join the EU/US/UK side and take an active part in support of Ukraine. One expression of disapproval might be agreement on the neutralisation Russia’s Security Council veto by their acquiescence in the physical removal of Russia’s delegation to the United Nations. If Russia’s delegate does not, or cannot, turn up then Russia cannot exercise its veto.

That dramatic step would demand a major breach of diplomatic protocol, but nuclear opprobrium could do it. There are sure to be other consequences. Russia may regard battlefield tactical weapons as “just a bigger bang” but few other states will see them that way.

For now, Russia’s raised alert status appears to be a move calculated to breed fear and apprehension among the mass of the West’s population, who live in happy ignorance of just how near the possibility of annihilation actually is. The 700mn people in Europe and the US have grown used to thinking that the Ukraine war cannot personally hurt them. As soon as the word “nuclear” hits the airwaves that feeling can vanish. Ms Truss might do well to learn more about the history of the nuclear calculus and to choose her words more carefully.