For years, the U.S. military has been fretting about the return of conflicts in which air defenses play a big role. If tensions escalate in  Syria, high-end American fighter jets might face off against Russia's top-of-the-line anti-aircraft missiles — and both sides will quickly learn who has the edge.

Russia began moving its high-end, ready-for-World-War-III air defense equipment into Syria early on in the conflict. In November 2015, a Turkish F-16 downed a Russian Su-24M near the Turkish border. Russia deployed an S-400 anti-aircraft radar and missile battery into the country. It's basically a truck with a big radar several more trucks with missiles capable of detecting and intercepting air targets at a range of 400 km, at least according to Russian descriptions of January 2015 intercept tests.