WASHINGTON — The Pentagon’s new focus on threats from China and Russia after almost two decades of fighting terrorism “too often rests on questionable assumptions and weak analysis,” an independent bipartisan commission concluded in a sharply critical report issued on Wednesday that challenges President Trump’s commitment to supporting a strong military.

Over all, the panel that was appointed last year by Congress praised the general direction of the National Defense Strategy that was issued in January by Defense Secretary Jim Mattis.

But it warned that projected budget shortfalls, overtaxed military forces around the globe and other risks were imperiling the plan, just as it was taking effect.

“America’s longstanding military advantages have diminished,” according to the wide-ranging, 98-page report, an advance copy of which was made available to The New York Times. “The country’s strategic margin for error has become distressingly small. Doubts about America’s ability to deter and, if necessary, defeat opponents and honor its global commitments have proliferated.”