The main focus of U.S. defense strategy has shifted from defeating violent extremists—terrorists—to opposing aggression by other great powers. Those other powers are understood to be China and Russia. Pentagon planners now spend much of their time figuring out how to deter Russian adventurism in Europe, and Chinese expansionism in Asia.
The general thinking is that Europe will be mostly an Army-Air Force show, because any Russian military move westward would occur on land, and the only seas close enough to facilitate Navy-Marine Corps participation (the Baltic and Black Seas) would be too dangerous to enter in a major conflict. So Washington is moving to expand the Army's presence in Poland and the Baltic states while increasing the number of stealthy Air Force and allied F-35 fighters in the region that can support ground operations.