This article discusses why Russia's looming Zapad 2017 military exercise with Belarus and in areas near the Baltic states.
It's all very interesting in a discussion of drones and tactics (and you know my opinion on "hybrid" warfare) and whatnot (yet don't neglect the maskirova aspect of the exercise, which Russia knows NATO will closely watch), but misses the point that the exercise is dangerous for just one basic reason.
The reason is that Russia is close to--and America and the majority of European powers are far from--the potential theater of war in Poland and the Baltic states.
And an exercise can shift into an actual invasion of NATO in the blink of an eye.
That's it.
Russia is weaker overall than NATO, but Russia can send in overwhelming force to grab territory in Estonia, Latvia, and Lithuania before NATO can react.
Part of this problem is that if Russia uses the large-scale military exercise to move significant numbers of troops into Belarus and just leaves them there in new permanent bases, the threats to Lithuania and Ukraine are expanded and a new threat to Poland is created.