Do you believe everything that was part of the Soviet Union belongs to Putin?
You as many other Americans have a skewed understanding of Russia thanks to the press and the neocons.
Russia is not a threat to the US. They are a third world country with A second world military. The very modest Russian economy does not produce the money necessary to conduct war which is why Russia is almost never at war with anybody. They can’t afford it. The Russian military is a bunch of conscripts lacking the will to fight as evidenced during the Crimea separation conflict.
Crimea is not historically part of the Ukraine. It has changed hands countless times. Again Putin is not going to give up his port on the Black Sea. He was afraid the Ukraine was going to renege on the lease after the overthrow of the elected Ukrainian President. That’s why he took the Crimea and he’s not giving it back and is none of our business to get involved in with provocative military posturing.
If Putin thought he could get away with it he would possibly try to take the Ukraine but he does not have the money for the protracted war of attrition that would emerge.
Russia is a pain in the spetznatz and a bully to its closest neighbors. That’s about it.
The best course of action for Trump to follow is to improve relations with Russia so he can exert influence over Putin. Putin very much wants to be a player and be accepted into the good old boys trade club with the West. His seat at the G8 conference is extremely important to him. The carrot and the stick is how you control Putin.