WASHINGTON—U.S. intelligence and military officials believe Kim Jong Un is a rational actor, a conclusion that for now is guiding Washington’s approach to the North Korean leader as he risks economic sanctions and military reprisals to build nuclear weapons and threaten rivals.

The assessment by the main components of the U.S. national security community has shaped their thinking toward North Korea in two major ways, U.S. officials said. It means they believe that Mr. Kim understands that any attack on the U.S. or its allies...