Russia’s assault on Ukraine over the past year has made it clear that President Vladimir Putin is out to reassemble as much of the former Soviet Union as he can, having once called its collapse “the greatest geopolitical catastrophe of the century.” It should be of great concern in the West that Bulgaria, a member of NATO and the European Union, is one of his targets.

In a September address in Sofia to members of the America for Bulgaria Foundation, Bulgarian President Rosen Plevneliev compared Moscow’s stealthy undermining...