Turkey and the US are heading for a high stakes crisis

Among several chummy meetings Donald Trump held with authoritarians at the Osaka G20 summit was one with Recep Tayyip Erdogan of Turkey. Mr Erdogan emerged insisting the US president had told him Washington would not impose sanctions over Turkey’s obdurate plans to buy an air defence system, the S-400, from Russia. Turkey’s president told local media the first shipments would arrive within 10 days. Yet for all the display of bonhomie in Osaka, the deal is a slow-motion collision between Turkey and the US that could turn into a train-wreck.

The US Congress is unlikely to be as sanguine as the president on sanctions. Washington has repeatedly made clear Turkey cannot buy both the F-35, the new stealth fighter jet being produced by the US and its allies — including Turkey itself — as well as the Russian missile system. It has warned that if, as a Nato ally, Mr Erdogan sides with Russia, Turkey will be hit by the Countering America’s Adversaries Through Sanctions Act. The US has already suspended training for Turkish pilots on the F-35 and held up early deliveries of up to 100 of the aircraft Ankara expects to purchase. It is right to do so.

The S-400 deal is a diplomatic triumph for Vladimir Putin, Russia’s president, and his campaign to undermine western cohesion. It disregards the need for interoperable weapons systems within Nato. If these missile batteries are deployed inside Turkey, that will enable Russia to acquire information about the F-35, due to become the alliance’s main combat aircraft. Turkey, like any sovereign state, is entitled to make its own choices on defence procurement. But as a Nato member, it is not entitled to punch a hole in the solidarity and security of the alliance.

That is true even if Mr Trump has shown scant regard for Nato and is fixated on American arms sales. The principle upon which the alliance is based — a sort of all-for-one and one-for-all — is cardinal.

Late in the day, the US offered Turkey its Patriot missile defence system as an alternative to the S-400. Moscow is offering Ankara a share in developing the next generation S-500 — though that prospect hardly trumps the transfer of technology already available to Turkey under the F-35 and other programmes.

At the core of this problem is Turkey’s vulnerability to Russia as a result of the former’s tattered Syria policy, which is also at the heart of its accumulated grievances with the US. Mr Erdogan has become beholden to Mr Putin over the past three years. Moscow salvaged Bashar al-Assad’s regime while Ankara backed a variety of Islamist groups trying to topple it. Since 2016, however, Turkey’s main aim has been to stifle a self-governing Syrian Kurdish entity on its borders, run by forces backed by the US in the fight against Isis. Turkey, by default, now finds itself in a shaky alliance with Russia and Iran in Syria.

If Mr Erdogan jilts the Russians on the S-400, Mr Putin may step up his offensive against the last rebel enclave, in Idlib in north-west Syria, where Turkey has a dozen military posts. And Turkey’s occupation of two enclaves around Idlib, part of its campaign against the Kurdish People’s Protection Forces, or YPG, that control much of northern Syria below its borders, is only possible with Russia’s transactional consent.

Turkey, however, wants to press on into the territory the YPG controls in north-east Syria under US air force cover. If Mr Erdogan really antagonises the US, not just Turkey’s membership of Nato or F-35 partnership will be in question. He would be unwise to put too much faith in Mr Trump’s chummy weekend words.