In his resignation letter, Mr. Mattis condemned Mr. Trump’s rejection of international alliances and emphasized the importance of challenging Russia and China, which he wrote wanted “a world consistent with their authoritarian model.” Read his letter.

International reaction: America’s allies in Europe and Asia indicated today that Mr. Mattis’s resignation and U.S. troop withdrawals would be watershed moments in Washington’s relations with the world.

News analysis: The Syria withdrawal is one of several announcements this week that demonstrate Mr. Trump’s “unswerving conviction that his political survival depends on securing his conservative base,” one of our White House correspondents writes.

Catch up: Mr. Mattis is the third member of Mr. Trump’s cabinet to resign or be forced out in less than two weeks. Here’s a look at the turnover at the top of the administration.

U.S. withdrawal shakes up the Middle East

President Trump’s decision to bring home the 2,000 American troops in Syria clears the way for Iran to expand its influence in the region and leaves Israel isolated.

President Vladimir Putin of Russia praised Mr. Trump on Thursday, saying, “Donald’s right, and I agree with him.” Moscow and Washington have supported opposite sides in the Syrian war.