In 2012 Obama famously said in an aside to Russian President Dmitry Medvedev that he would "have more flexibility" after his next (and last) presidential election. This was at a nuclear security summit in South Korea. Since then he has reduced our military capabilities and seen Putin invade Crimea.
If Putin goes further, as he has threatened to do, the legal obligations of the United States per NATO can come in to play. So far, some people say Obama looks weak, but his rhetoric vis a vis Crimea and Ukraine can be disregarded. What if he's actually not so much weak as just cooperative? What if he's not acquiescing to unwanted aggression but accommodating it because it fits his own world view? Was that what he meant by "flexibility"?
The test will come if the U.S. is required to act in defense of its allies.
If only there were a way for the U.S. to be oil-independent.