Mr. Trump determined that imports of foreign cars and car parts were causing harm to the American automobile industry and threatening national security and said other countries had 180 days to address the problem through trade agreements with the United States.

If agreements “to address the national security threat” are not reached, Mr. Trump said he would determine “whether and what further action needs to be taken,” a step that could include imposing 25 percent tariffs on foreign cars.

The decision offers a temporary reprieve to global automakers and auto suppliers, which had been bracing for punishing tariffs on the more than 8 million cars imported into the United States each year. But it sets up a tense six-month period for the White House to reach trade deals with other nations that have already been complicated by mistrust and disagreements.