Colonel Sebastien Vallette swings back in his desk chair, minutes after landing his Mirage fighter on a Soviet-era runway. A detailed map on his wall shows Lithuania, Latvia and Estonia, flanked by bold warnings: "Do Not Enter Russian Airspace". For six years, NATO allies have taken turns patrolling the airspace of the three Baltic states, minnows which won freedom from the crumbling Soviet Union in 1991 and have testy ties with Moscow. Vallette and six other French flyers under his command are the current eyes in the sky. "The point about airspace sovereignty is to know who's doing what in...