WASHINGTON — President Trump on Tuesday fired his third national security adviser, John R. Bolton, as their positions on major foreign policy issues clashed, most recently on pursuing a peace plan with the Taliban.

The two men have regularly been at odds over how to take on major foreign policy challenges facing the United States. Mr. Bolton, a longtime national security hawk, held some views that contrasted with Mr. Trump’s, favoring sanctions and pre-emptive military action against some countries even as the president pursued diplomacy.