Defying US threats of sanctions, Turkey last week test-fired at least one missile of its Russian S-400 air defense systems in what was ostensibly a show of resolve to activate the systems, but Ankara might be eyeing other objectives as well.

While Ankara has neither confirmed nor denied the six-hour test, an anonymous US official told Reuters that Turkey did test the S-400 system. Turkish authorities had issued an aviation notice earlier this month that unspecified missiles would be fired near the Black Sea city of Sinop in mid-October. Experts analyzing footage of a missile launch in the area Oct. 16 have identified the projectile as an S-400 missile of the 40N6E type, which has a range of 400 kilometers (248 miles), based on the engine ignition type and the angle and route of the smoke. Two other tests were expected to take place in the same area by Oct. 19.

The test, which reportedly lasted about six hours, represents the most crucial phase on the way to the activation of the S-400 systems — a red line drawn by the United States — following earlier drills in November 2019, when Turkey used US-made F-16 and F-4 fighter jets to test other components, most likely radars, at an air base outside Ankara, where the four S-400 batteries have been kept since Russia began deliveries in July that year.

Now, the last step to watch before a full activation will be the official assignment of the S-400 operators to the Turkish air force’s combat structure, probably on the level of regiment, and their operational deployment to their places of duty.