Chuck Hagel became the first political casualty of the crises that have beset US foreign policy this year when President Barack Obama announced his defence secretary’s resignation on Monday, sparking a major post-election shake-up of his national security team.

Mr Obama has faced considerable pressure to reshuffle his senior national security officials against a backdrop of widespread criticism that the administration has fumbled its response to crises from Ukraine to Iraq.

Mr Hagel, whose resignation was announced on Monday morning by Mr Obama, is to remain at the Pentagon until a successor is confirmed.

The president brought Mr Hagel, a senior former senator, to the Pentagon in 2013 with a brief to manage the withdrawal from Afghanistan and to build political support for cuts to the defence budget. A Vietnam veteran who enlisted for the war and was awarded two Purple Hearts, Mr Hagel had considerable credibility with the rank-and-file.

However, his low-key approach has been criticised at a time when the US has been pulled back into fighting in Iraq, against the Islamist militants of Isis.

Reflecting a commonly held view in the Pentagon, Mr Hagel has privately pushed for the administration to do more to confront the Assad regime in Syria as part of the anti-Isis campaign. Mr Hagel has acknowledged that the air strikes against Isis in Syria could be boosting the position of the Assad regime forces.

Military officials have also complained that the White House is eager to micromanage operations against Isis.

Speaking in the White House, Mr Obama described Mr Hagel as an “exemplary” secretary of defence who showed “class and integrity”.

The resignation leaves Mr Obama looking for his fourth defence secretary. One widely mentioned potential successor is Michèle Flournoy. She was the third-ranking official at the Pentagon in Mr Obama’s first term and would be the first woman to lead the defence department.

Other names discussed include Ashton Carter, former deputy secretary of defence, and Jack Reed, a Democratic senator.

The departure of Mr Hagel threatens to set up a stand-off with the new Republican-controlled Congress over his successor. The conservative senator Ted Cruz said at the weekend that Republicans should respond to Mr Obama’s executive order on immigration by refusing to confirm his nominees. However, the Republicans would face considerable pressure not to leave the Pentagon leaderless while the US is at war.

Mr Hagel’s confirmation hearing in early 2013 was rocky with many of his former Republican colleagues in the Senate turning against him, and some pro-Israel groups sharply criticising his views on the Middle East.

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