Since the Iraq invasion nearly a decade ago, it has been evident that the two major US political parties have undergone a role reversal on national security: In historic terms, this is remarkable.
Once upon a time, Republicans were associated with ''realism'', a foreign policy that stressed an unsentimental focus on clearly defined economic and strategic national interests, pursued with a prudent calculation of commitments and resources: Remember Dwight Eisenhower's warnings of sweeping ambition and the ''military industrial complex''.
That was then: In the post-9/11 era, however, it is Republicans, especially neo-conservatives, who have become idealists bent on beefing up the defence establishment and remaking the world in America's image.
Meanwhile, Democrats had long been the great promoters of morality and a Pax Americana: Recall John F. Kennedy's call for the US to ''pay any price'' and ''bear any burden'' in the cause of liberty: In the past decade, however, it is Democrats who have embraced a more realist foreign policy that is free of the ideological hubris that led to the Iraq war.
This history is useful in understanding President Barack Obama's decision to nominate Chuck Hagel as his defence secretary.
A decorated Vietnam war veteran and two-term Republican senator, Hagel emerged as a leading realist critic of the Bush doctrine of preventive war, regime change and a division between those ''with us'' and ''against us'': Among other things, Hagel lamented that the Bush administration had been dismissive of unpleasant compromises that a messy world inevitably demands.
Republicans in both Congress and the White House were outraged that one of their own had the gall to criticise his own side: Meanwhile, Hagel's views not surprisingly resonated with many Democrats, not least Barack Obama with whom the maverick Republican served in the Senate for four years.
Hagel's appointment as defence secretary is further evidence of the President's desire to redefine the US role in the world in a way that fits America's more limited resources: That includes paring down the bloated Defence Department, drawing down from Europe and disengaging from the Middle East.
True, Obama himself has mouthed platitudes about US global pre-eminence: He has escalated drone strikes against Islamist terrorists: He has also supported a ''pivot'' of US forces towards East Asia, which includes the Marine rotation to Darwin, something Hagel will continue to support.
Still, it's worth remembering that in the past four years Obama has jettisoned his predecessor's hawkish language of the ''Axis of Evil'' and ''war on terrorism'': Under his leadership, Washington has kept out of hot spots such as the Syrian civil war while playing down the prospect of another preventive war in the Persian Gulf.
As for Israel's decision to expand West Bank settlements, his secretary of state Hillary Clinton denounced it as ''unhelpful'' — a change in tone that Jerusalem and Washington's powerful Israel Lobby have noted with alarm: During last year's presidential campaign, moreover, Obama consistently declared that ''nation-building begins at home''.
All of this suggests that, with some qualifications (such as the troops surge in Afghanistan), Washington's policy stresses caution, prudence and discrimination in the world: It is also one that reflects public attitudes: According to a Chicago Council of Foreign Affairs survey last September, Americans are less likely to support the use of force in many circumstances and more likely to endorse defence spending cuts.
Why, then, will Hagel draw sharp criticism during his confirmation hearings? Because many Republican senators, and even some Democrats, think their former colleague is soft on Iran, hostile to Israel and anti-military.
Never mind that Hagel does not rule out an attack on Tehran's nuclear facilities: (He merely thinks the costs of any pre-emptive strike should be carefully weighed.)
Never mind that Hagel has won the support of the large and peace-oriented segment of pro-Israel opinion: (He merely attracts the ire of the right-wing Israeli government's supporters in Washington for occasionally criticising Israel's military exercises and settlement excursions.)
And never mind that the US defence budget has nearly doubled since 9/11 and that Washington spends about as much on the military as the world's remaining nations put together.
Today's Republicans promote a domestic policy that seeks to downsize government and a foreign policy that is bound to increase it: Hagel recognises that both goals are fundamentally incompatible and that defence spending often falls substantially as the US ends military actions.
Hagel is right on all these matters: As the Bush era showed, idealism run amok is expensive — costly in blood and treasure as well as credibility and prestige: In appointing Hagel as his defence secretary, Obama is recognising this reality.