John R. Bolton is hardly known for sugarcoating his views. He once derided the United Nations by citing its 38-story headquarters in New York. “If it lost 10 stories,” he said, “it wouldn’t make a bit of difference.”

As it happened, Mr. Bolton went on to serve as the United States ambassador to the United Nations, perhaps the most undiplomatic of diplomats. But if President Trump wanted a national security adviser who would match his blunt, hard-edge, confrontational approach to the world, then Mr. Bolton fits the bill.

With his trademark bushy gray mustache and his take-no-prisoners style, Mr. Bolton positioned himself to the right even of the foreign policy veterans who emerged from President George W. Bush’s administration, a hawk among hawks, a hard-liner who thrills conservatives and chills moderates and liberals. From his perch on Fox News, he has impressed Mr. Trump with a muscular vision of American power and a dark assessment of America’s adversaries.

When he takes over as Mr. Trump’s third national security adviser in 14 months, Mr. Bolton will almost surely encourage Mr. Trump’s instincts against diplomatic agreements both consider weak and unwise. He shares the president’s derisive opinion of the Iran nuclear deal and will presumably prod him to scrap it when a May deadline arrives. He likewise takes a dim view of international agreements like the Paris climate change accord, from which Mr. Trump announced last year that he would withdraw the United States. He has called the “two-state solution” for Israel and the Palestinians dead.