WASHINGTON — U.S. Defense Secretary Chuck Hagel resigned on Monday, leaving under pressure as President Obama faces critical national security challenges, including fighting Islamic State in Iraq and Syria and revising plans to exit Afghanistan.

It was the first major change to Obama’s Cabinet since Democrats were routed in midterm elections three weeks ago, and Republicans, who now control both houses of Congress, are looking for a new approach from the White house.

Hagel was appointed less than two years ago as Obama pushed his signature program of winding down wars in Afghanistan and Iraq, a process now being upended with U.S. re-engagement in Iraq and greater military cooperation with Kabul.

He had privately expressed frustration to colleagues at the administration’s strategy toward Iraq and Syria and at his lack of influence over the decision-making process, a source familiar with the situation said.

Officials said publicly the decision for Hagel to leave was mutual but privately others said he was forced out. “There’s no question he was fired,” said one source with knowledge of the matter.

Hagel raised questions about Obama’s strategy toward Syria in an internal policy memo that leaked this fall saying the president’s policy was in jeopardy due to its failure to clarify its intentions toward Syrian President Bashar al-Assad.

Obama has insisted the United States can go after Islamic State militants without addressing Assad, who the United States would like to leave power.