Twenty-nine US senators have signed a letter to President Biden calling on him to block sales of 40 new F-16 aircraft and upgrade packages for another 79 (worth a reported $20bn) to Turkey.

The letter explicitly links the objection to Turkey’s refusal (so far) to ratify Sweden’s entry to Nato. The letter was bipartisan, and led by Democrat Senator Jeanne Shaheen and Republican Thom Tillis. In the House of Representatives two supporters of the Greek lobby tried unsuccessfully to insert a legislative ban last summer.

Ankara initially has blocked the accessions of both Finland and Sweden to Nato until Kurdish dissidents resident in both states are extradited to Turkey. The three countries met in Madrid in June 2022, where they agreed a memorandum of understanding (MoU) in respect of extraditions. Under this Finland and Sweden committed not to extend support to three Kurdish organisations – the YPG and its associate party the PYD, the Fethullah Terrorist Organisation (which Ankara considers to be the terrorist arm of Fethullah Gulen) and the PKK.

Finland and Sweden also committed to amend their criminal codes to widen the definition of terrorist crimes.

In respect of the extraditions the memorandum was much vaguer. Finland and Sweden merely undertook to “address” Turkey's extradition requests “expeditiously and thoroughly”, but while complying with the 1957 European Convention on Extradition. Article 3 of that convention excludes “political offences” from the treaty (with the requested party deciding what a political offence is). The memorandum therefore gave Turkey some comfort as to future actions by Sweden and Finland but failed to deliver Ankara’s main (or at least its stated main) objective.

The MoU also fell short of Ankara’s desires by failing to define the YPG as a terrorist organisation in Sweden and Finland (it merely obliges them not to support it). There are multiple other obstacles to extradition in other fields of refugee law and the laws protecting human rights which also stand in Turkey’s way, and which the memorandum ignored.

As a result, Ankara is still refusing to ratify the entry to Nato of either Nordic country. It appears to be less hostile to Finland’s entry, which may be in part because the extradition issue is less pressing (most of the targeted individuals are in Sweden) and/or because Helsinki has allowed the export of some military-grade steel from Finland to Turkey (ironically by a Turkish-owned company).

However, as things stand Turkey’s veto is holding. The Atlantic Charter has no provision allowing Nato to sidestep a veto. There is equally no provision to expel a non-cooperative member. Ultimately, if Turkey stands on its veto rights (which would probably also be exercised in respect of other candidate members, such as Ukraine or Georgia) and the rest of Nato members agree to expand the organisation their only course of legal action may well be to leave the organisation unilaterally (permitted under the treaty) and form a new Mark 2 Nato without Turkey and with the new desired members.

The enormous disruption and difficulty contained in that course may be the reason that Washington is allowing an F-16 veto to enter the narrative – F-16s in return for Sweden and Finland being the message.

The Senate letter is little more than a line in the sand. In order to block the sale both Senate and the House need a two-thirds vote to veto it. The Senate looks too deadlocked to reach that threshold, especially against the headwind of a generous flow of lobbying cash from Lockheed Martin. The Greek lobby in the House may be vociferous but is unlikely to win the argument either. Its attempt last summer to install a blocking provision in the 2023 Defense Authorization Act, for example, went nowhere.

Which leaves the decision essentially with the Administration. Here the pressures are finely balanced. A refused export licence would likely drive Ankara further towards the welcoming arms of Russia, which has already sold its highly capable S400 air defence system to Turkey and is also building the latter’s new nuclear power plant (NPP). Under pressure Ankara might start buying Russian aircraft as well.

If Ankara abandons the F-16 development track a large component of Washington’s influence structure over it would start to die. The US would also lose an element of veto over the use of Turkish air power provided by control of the supply of spare parts and compatible air-launched weapons.

Also relevant is Ankara’s current bellicosity towards Greece – now largely a US client state. Turkey has been semi-detached from Nato for a decade. Refusal for the F-16 sale might push it far enough away to make space for aggressive action towards the handful of Greek islands that abut its west and south coasts.

Greece is not the only possible target for Turkish aggression. The frozen conflict in Cyprus permanently contains the possibility of re-igniting, a possibility increased by Ankara’s claims over an enlarged Exclusive Economic Zone under its Blue Homeland programme. Provocation of Turkey contains substantial and visible risks of geopolitical blowback for Nato members and US interests.

That may be why, so far at least, the Administration has consistently supported the planned F-16 sale, though occasional senior voices in Washington (including the recently retired head of Centcom, General Votel) are arguing that the sale should be vetoed if Ankara fails to give hard undertakings on how F16s might be used (i.e. not against Greece).

In order to get what it wants – Sweden and Finland in, and F16s sold – Washington has to propound a credible threat to embargo Turkey’s F16s. Ankara may decide that this is the moment to swap its warplane solution to a Russian source. If that happens then fur will fly in Washington.

It is too early to say on which side the two parties’ decisions will fall. As usual, Washington’s multitude of global agendas present it with difficult competing decisions, but on balance a sale looks more likely than a veto, which tends to make the embargo threat an empty one.