The concerns prompting the new strategy were previewed by Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, at his Senate confirmation hearing in July 2015: “If you want to talk about a nation that could pose an existential threat to the United States, I would have to point to Russia. And if you look at their behavior, it’s nothing short of alarming.”

China worries some Pentagon officials even more than Russia. A recent study by the Rand Corp., titled “War With China: Thinking Through the Unthinkable,” warned: “Improvements in Chinese military capabilities mean that a war would not necessarily go the way U.S. war planners plan it. Whereas a clear U.S. victory once seemed probable, it is increasingly likely that a conflict could involve inconclusive fighting with steep losses on both sides.”

Top Pentagon officials say that because of Russian and Chinese advances, the U.S. military’s “overmatch” has diminished. Planners can no longer guarantee a president that the United States could prevail in the early days of a conventional conflict; they fear that the United States might lose “escalation dominance” — meaning, basically, the ability to call the shots — in a future confrontation.