President Trump said on Thursday that he had picked John R. Bolton, a Fox News analyst and a former ambassador to the United Nations, as his next national security adviser, choosing a longtime brash, conservative hawk to help shape his foreign policy operation.

Mr. Bolton, 69, has been in the public eye, and in Washington, for 30 years. Outspoken and never one to back down from a debate, he served in the administrations of the past three Republican presidents, mostly in the State Department, and has elevated his profile in recent years. Some of his views were once seen as out of the political mainstream — his harsh criticism of the United Nations, for example — but his blunt, in-your-face approach now resembles that of Mr. Trump.

Over the years, Mr. Bolton’s uncompromising views on foreign policy have both raised concerns and grabbed headlines. Here are some examples.

‘The End of North Korea’

Mr. Bolton, who is scheduled to start at the White House on April 9, will take over as a top adviser to Mr. Trump during a critical time. Mr. Trump recently accepted an invitation from North Korea’s leader, Kim Jong-un, to meet for negotiations over its nuclear program. That meeting is expected to take place sometime within the next month and a half.