What I would do to let Russia invade Europe and gain land quickly is a sort of purge in the US military. Maybe a purge of generals or brass. Yeah a purge could work, only if there is enough generals to purge. Maybe force the generals to lick your shoes in loyalty and forget the constitution, do an purge of leadership event that would be blamed for the lack of US reaction and response to a Russian invasion. Yeah that's it, so when the Russians take the Baltic, US response would be blamed on bungling the defense of Europe. You know how the military had the bungled response on 9/11 due to the military exercises happening.

A scheme such as this:

https://youtu.be/iuyemPUXSz0?feature=shared&t=879

So then Europe is more vulnerable. Though the generals have to resist Trump for this to possibly work.

The likelihood of this working is slim. Though there are other ways of weakening US defense of Europe before a planned Russian invasion of Europe.