Tom Switzer is research associate at the United States Studies Center at the University of Sydney and editor of The American Review.

When Chuck Hagel appears before the US Senate Armed Services Committee overnight, he is likely to face tough questions about his past positions on Iran (he prefers containment to prevention), Israel (he once criticised the so-called 'Jewish lobby') and defence spending (he thinks the 'bloated' Pentagon should be 'pared down').

But the fight over his confirmation as defence secretary has obscured the philosophical meaning of his nomination: President Obama's attempt to define a new US world role that reflects America's changed circumstances and more limited resources.

In the wake of the financial crisis and the Iraq and Afghanistan wars, Americans are rediscovering the costs and limits of the use of force. Consequently, the President appears to be in the process of putting into practice the tenets of what is loosely described as realism, a foreign policy that stresses an unsentimental focus on clearly defined economic and strategic national interests, pursued with a prudent calculation of commitments and resources.

Of course, American realists come in different shapes and sizes: some are left-liberals and Democrats; others are conservatives and Republicans; some believe the US should seek security by exploiting the balance of power in an emerging multipolar or pluralistic world; others think Washington should seek security through global hegemony.

What unites realists, however, is the unapologetic defence of the national interest, which involves a dispassionate consideration of circumstances, of costs and benefits, and ends and means.

Two realist themes appear to define a second-term Obama Administration. One is that it is time for the US to focus on 'nation building at home', something the President has repeatedly stressed in the past year. The other is the vague, subtle recognition that American unipolarity – which began with the collapse of Soviet Communism – is being replaced by a world populated by powerful new players.

This means that, with the tide of war receding, as the President noted in his inaugural address, the new administration will emphasise caution, prudence, balance, modesty and proportionality in dealing with adversaries and competitors. It also means that, depending on the circumstances and the nature of its interests, the US will pick and choose where to commit its considerable weight in the world.

Hagel's hawkish opponents appear to think of foreign policy in terms of sweeping doctrines that champion general, binding principles and rules of conduct that must be followed consistently. Not surprisingly, they think the views of the defense secretary-designate are 'out of the mainstream of (foreign policy) thinking,', as Republican Senator Lindsay Graham argues.

If anything, it is the decorated Vietnam veteran who better represents public attitudes, and it is his neo-conservative critics who are out on the fringes.

Simply put, the American people are suffering from foreign-policy fatigue. For seventy years – first against fascism, then communism, and more recently Islamist terrorism – they supported and sustained a defence commitment of the most intense and comprehensive kind. Everything else was subordinated to it and all sorts of domestic concerns were neglected. As America withdraws from Afghanistan – a war that has lasted longer than both world wars combined – there is an overwhelming feeling that it is time for a respite from global responsibilities.

That was the message of last September's survey of American public opinion and US foreign policy published by the Chicago Council on Global Affairs. Among other things, it found that Americans are now less likely to support the use of force in many circumstances and are more likely to endorse defense spending cuts; that majorities oppose either a unilateral US pre-emptive strike on Iran or even an attack authorised by the UN; and that more than half of the 'millennials' — those between the ages of 18 and 29 – believe the US should 'stay out' of world affairs.

That is why foreign policy was the dog that did not bark in last year's presidential election. And it is why the President dedicated only a few sentences to the subject in last week's inaugural address.

Until recently, a huge gap had opened between America's global pretensions and its ability to finance them. But Obama seems intent on matching aspirations and resources — or, in the distinguished journalist Walter Lippmann's words, to bring commitments and power into balance.

In the past four years, as I recently observed in the Fairfax press, the President has jettisoned his predecessor's doctrine of preventive warfare, aggressive unilateralism and a clear division between those 'with us' and 'against us'. Washington has kept out of hot spots such as the Syrian civil war while playing down the prospect of a pre-emptive strike on Iran's nuclear facilities.

This new strategy does not stop Obama from escalating drone strikes against terrorists, nor will it derail his administration's 'pivot' of US forces towards East Asia. But it does allow him to reorder priorities in favor of domestic concerns.

Hagel's critics might dismiss such realism as a precursor to a new isolationism. But it should be seen as protecting the US from over-commitment and the type of ideological hubris that led to the Iraq invasion a decade ago.

Photo by Flickr user Secretary of Defense.