The purpose of NATO, per its first Secretary General: to “keep the Soviet Union out, the Americans in, and the Germans down.”
The thing the American ruling class fears most (at least until now, the rise of China) is an alliance between Russia and Germany, and they’ll go very far to prevent that. The purpose of the continuing American occupation of Germany is not to keep Russia from invading Germany, there’s zero risk of that.
Oh yeah, because of all the great Eastern European alliances Russia is known for. /s
While that would suck, why on gods earth would democratic Germany align itself with Russia? Looking for another Berlin Wall? They really loved that the first time.
There's no fear of either an alliance, or invasion, thanks to American and other Nato troops. But continue live ingin your ivory education tower - that's what we fight for after all.