“Is that sufficient capacity and capability to do the various national strategies?” General Milley said of current troop levels to a group of foreign policy experts at the Council on Foreign Relations in New York on May 3. “We think it is. But the real hard question is what happens if one of these other contingencies were to go off that.” He was referring to potential conflict on the Korean Peninsula, or with a large power like China or Russia.

President Obama has begun to respond to the re-emergence of Russia as a strategic threat. The White House has quadrupled the budget for military spending in Europe in 2017, to $3.4 billion. As part of an effort to deter Russia, the United States will provide additional weapons and equipment to American and NATO forces in Europe, to ensure that the alliance can maintain a full armored combat brigade in the region at all times.

But other threats are not going away, forcing Army officials and Pentagon planners to figure out how to adapt America’s military strategy to the new global reality. Army officials are trying to balance the military’s responsibilities in Africa, the Middle East and South Asia while relearning how to fight higher-end, great-power conflicts, as well.

One possible solution, General Milley said, could be to increase training days for the National Guard. Its members now train 39 days a year, which allows them to be ready to deploy within four months if called up.

“I do not think we will have the luxury of four to five months lead time if a significant contingency comes up,” General Milley said.

This week, his focus is on Africa, which has increasingly become a battleground in the West’s war against militant Islam.

In Central Africa, American service members are working with militaries from Nigeria, Chad, Cameroon and Niger to counter Boko Haram. American military officials say that Boko Haram has begun to collaborate with the Islamic State, which is also known as ISIS or ISIL, and that the terrorist groups — two of the world’s most feared — could be working together to plan attacks on American allies in North and Central Africa.