President Biden and his team saw China as the one nation with the intent and capability to displace American primacy — and crafted policies to defend U.S. power.

President-elect Donald J. Trump has made big promises on Ukraine, Iran, China and crises around the globe. But he will have to make difficult choices.

The White House scrambled to get a message to President Vladimir V. Putin of Russia last year after U.S. intelligence agencies said a Russian military unit was preparing to send explosive packages on cargo planes.