[M T]

I wonder whether Russia could undertake a new invasion within the next 4 years. At that point, the leadership of the US has a good chance of changing and its commitment to NATO may change.(*)

Granted, the world has seen how fickle the US can be and needs to be better prepared for that fickleness. 
 
(*)This is assuming the US continues as it has for nearly 250 years.

[M T]

I wonder whether Russia could undertake a new invasion within the next 4 years. At that point, the leadership of the US has a good chance of changing and its commitment to NATO may change.(*)

Granted, the world has seen how fickle the US can be and needs to be better prepared for that fickleness. 
 
(*)This is assuming the US continues as it has for nearly 250 years.