This line of reasoning keeps popping up and something about it bothers me - why go to war when you can get what you want in other, cheaper ways? It seems likely the correlation is real but so far no one has adduced any reasons to assume the causation actually goes the way they assume.

Whoever was POTUS played no role in the timing of 2014. Putin invaded Donbas in 2014 in response to a revolution in Ukraine that ousted the unpopular Russia-aligned Yanukovych. Not because Obama was POTUS or because Trump wasn't POTUS.