US Defense Secretary Chuck Hagel unexpectedly resigned this morning (AEST), the apparent victim of heavily criticised Obama Administration foreign-policy failures and brooding discontent within the White House.

Hagel, one of just two Republicans in the Obama White House, gave no definitive reason for his sudden decision to leave the Pentagon after less than two years in the job. But there is strong speculation he was fired. He is the first cabinet-level casualty since voters delivered a stinging rebuke to President Obama at the mid-term elections earlier this month.

Hagel had a difficult time politically at the Pentagon almost from the start. After his embarrassingly inept confirmation hearings in January 2013, when he showed he did not even know the US policy on Iran, Hagel was approved by the smallest margin in the history of US defence secretaries. Nearly all his fellow Republican senators voted against him.

The quick succession of crises this year — from the Russian incursion in Ukraine to the march of Islamic State terrorists in Iraq and Syria to the outbreak of the Ebola virus — resulted in Hagel being criticised as a poor leader of the military and President Obama being frequently portrayed as a weak commander in chief.

Hagel's forced departure from the Pentagon is a troubling signal of President Obama's continuing problems in assembling and running an effective Cabinet with which he feels comfortable. During his two terms, Obama will have had at least four defence secretaries. (By contrast, George W Bush and Ronald Reagan had two confirmed secretaries while Bill Clinton had three Pentagon heads.)

As a senator, Obama admired and benefited from Hagel's stature as a defence thinker, a decorated Vietnam veteran and one of the senior Republican critics of the Iraq war. The re-elected president remembered this when the time came to name his new Pentagon head in the (northern) winter of 2012-13. But it now appears Obama did not really know what he was getting in Hagel. The learning curve was steep for the president, fatal for the defense secretary.

Among other things, Hagel was an indifferent manager and unskilled bureaucratic infighter. Even those of us who welcomed Hagel's appointment were disappointed with his slowness to master the Pentagon bureaucracy and his failure to project a new realist vision for US defence and security policy.

Moreover, although he initially shared the President's instinctive realism and caution about indiscriminately throwing America's weight around in the world, it is widely understood Hagel clashed with the White House over Obama's failure to prosecute the case against Assad's regime in Syria more aggressively. He was seen as outside the President's tight bunker of advisers that has centralised foreign-policy decision-making more than any president since Nixon.

If this is true, then Hagel merely reflected the thinking of his immediate predecessor Leon Panetta as well as the former secretary of state Hillary Clinton, who aired their criticisms of the President in their memoirs this year.

Hagel's successor will be subjected to grueling confirmation hearings led by veteran hawkish Republican Senator John McCain. (At this stage, the favourites are Ash Carter and Michele Flournoy) He or she will then confront the challenge of adapting an expensive military machine to the needs of a changing post-9/11 world and keeping the confidence of a President still feeling his way on foreign policy.

With two years left in his term, it is clear that whatever Obama's cautious and prudent campaign themes of 2008 may have promised, his foreign policy has taken a decidedly a more interventionist turn. From Ukraine to the Middle East to the so-called pivot in East Asia, the Obama Administration has recently begun to adopt a more assertive leadership role in the world. The extent to which the US continues along this path will have a lot to do with Hagel's successor.