Lithuania, a tiny Baltic nation bordering Russia, and long under the thumb of the Soviet Union, has been wary of its large neighbor to the east. The small nation can hardly provide for its own air defense needs. Accordingly, NATO nations rotate the duty of providing additional air defense assets to Lithuania.
Currently, the US Air Force is a part of that rotation. And given the events in Crimea and Ukraine, the US is anxious to remind Russia that its territorial ambitions have limits.
Similarly, the US has deployed F-16s to Poland and to Romania as well.