In Europe, a resurgent Russia, wary allies and a military mission to be defined

America’s next president will have to pass judgment on a military strategy that has centered for the past two years on building up forces in Europe, after the United States had been steadily cutting back since the end of the Cold War. The increases come in response to concern about a resurgent Russia, which Pentagon brass has called a top threat after Moscow’s 2014 intervention in Ukraine.