It's getting harder for NATO to hide that Turkey's becoming 'a thorn in everyone's side'

After months of warnings from the US and other NATO members, Turkey tested its S-400s last week, putting the Russian-made air-defense system, deployed to an airbase near Ankara, up against US-made F-16 jets.

The S-400 was delivered to Turkey this summer but hasn't become operational, which Ankara has said will happen in April.

But the system has been a point of contention since before it arrived in Turkey, and despite NATO leaders' assertions otherwise, the S-400 is one of many issues casting a pall over the alliance as it turns 70 years old.

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