SINGAPORE - How the world twists, then turns!

Two years ago, a Turkish Air Force F-16 shot down a Russian Sukhoi-24M fighter it said had intruded some 2km into its airspace for less than 20 seconds.

On Monday, Turkish President Recep Tayyip Erdogan, with his Russian counterpart Vladimir Putin standing beside him, told the media that his country would buy the S-400 - Russia's most advanced long-range anti-missile system.

"Our colleagues will come together this week to conclude this work," Mr Erdogan said.

The deal is a Russian dagger in the heart of Nato, whose only Muslim, Middle Eastern nation is Turkey. Nato's military chief has previously warned that Turkey, which has been a Nato member for more than 50 years, might face restrictions on continued access to its mutual air defence system.

While the weapons purchase has been talked of for some months, it has special significance for the Muslim world, coming a week after US President Donald Trump said he would move the American embassy from Tel Aviv to Jerusalem, an announcement that has aroused widespread ire in the region and elsewhere.

Turkey found its entry into the European Union denied after Germany decisively called for an end to accession talks with Ankara.

Mr Erdogan, who has made several moves that could be perceived as flirting with fundamentalist Islam - the country has a secular Constitution - is clearly making a series of policy adjustments that now has moved into the strategic sphere.

US policy in the Middle East, particularly in Syria and Iraq, has been clashing with Turkish interests for some time.

For its part, Russia has been angered by Nato's eastward expansion and sees an opportunity in gaining influence with a significant military power straddling the Middle East and Europe. This was why Moscow quickly buried the hatchet with Ankara after the Sukhoi downing in November 2015.

Mr Putin's travels this week also took him to Syria, where he announced a substantial reduction in Russian military presence after the defeat of Islamic State in Iraq and Syria, and also to Egypt.

Before the impending sale to Turkey, Russia is known to have sold the sophisticated S-400 only to India, once a close ally, and to China, with which it is increasingly coordinating global policy.