The Obama administration helped ignite that war by attempting to yank Ukraine out of Russia's orbit and into the Western security and economic sphere. Working alongside the European Union, Washington fanned mass street protests that led to a violent putsch against Kiev's elected government in February 2014. Moscow responded by annexing (or, depending on your point of view, reunifying with) Russian-speaking Crimea, which is also headquarters of Moscow's Black Sea Fleet, and backing pro-Russia separatists in the eastern regions of Donetsk and Luhansk.
Since then, the two sides have fought to a bloody stalemate. Besides killing thousands of civilians, the war has sunk Ukraine's economy and fostered rampant corruption. U.S. and E.U. sanctions have dragged down Russia's economy and derailed cooperation between Washington and Moscow in other theaters. Rising tensions between NATO and Russia have greatly raised the odds of an accidental military confrontation between the world's two largest nuclear powers.