It’s generally assumed that NATO member nations are going to import their weapons from NATO arms dealers, which pretty much always means the United States, with a few small deals for Britain and France. Turkey however, is going a different way on air defense, signing a deal to buy a $2.5 billion S-400 system from Russia.

Since the whole rest of the NATO alliance is night and day building up military forces on the Russian frontier, such a purchase is raising more than a few eyebrows, and not just because they aren’t going to buy a more expensive alternative from Raytheon or Lockheed.

The deal means Turkey’s new air defense system won’t be compatible with the rest of the alliance for the purposes of integration, though the S-400 is widely considered among the most advanced in the world. Russia will directly provide two S-400 batteries on the deal, and will also produce two more batteries within Turkey in the future.

This isn’t the first time Turkey looked to an alternative supplier outside NATO, as they very nearly bought missile systems from a Chinese company, before the US convinced them not to because the company had sold similar systems to the Iranians.

That Turkey still went outside the alliance may reflect the continued tension with both the US and Western Europe, as well as the Erdogan government’s interest in staking out a more independent foreign policy, as the Russian systems likely won’t come with the same restrictions on deployments as a Patriot missile battery.