The Trump administration is planning to tell NATO allies this week that it will shrink the pool of military capabilities that the US would have available to assist the alliance’s European nations in a major crisis, three sources familiar with the matter said. Under a framework known as the NATO Force Model, the alliance’s member countries identify a pool of available forces that could be activated during a conflict or any other major crisis, such as a military attack on a NATO member. While the precise composition of those wartime forces is a closely guarded secret, the Pentagon has decided...