For the last 20 years we have lived amid the furious clanger of war - and debates over how to wage it. The intense and urgent clashes in the 1990s over “humanitarian intervention” gave way to pitched battles over “regime change” and “democracy promotion” after 9/11, and then to arguments over “counterinsurgency strategy,” a new battle for hearts and minds, as Barack Obama ramped up the war in Afghanistan.

The foreign policy debate has often felt like an ideological cockfight. And now, although we have not yet realised it, that era has come to an end.

For proof, you need look no further than the Pentagon’s new “strategic guidance” document, issued last month in the wake of Obama’s pledge to cut $485 billion from the defence budget over the coming decade. It repeats many of the core objectives of recent American national security strategy: defeat Al Qaeda, deter traditional aggressors, counter the threat from unconventional weapons.

But it also states, “In the aftermath of the wars in Iraq and Afghanistan, the United States will emphasise non-military means and military-to-military cooperation to address instability and reduce the demand for significant US force commitments to stability operations.” It goes on to note that “US forces will no longer be sized to conduct large-scale, prolonged stability operations.”

With this paragraph military planners signalled an abrupt end to the post-9/11 era of intervention. Only a few years ago the wars in Iraq and Afghanistan - wars of occupation, nation-building and counterinsurgency - looked like the face of modern conflict. Now they don’t. Americans don’t believe in them and can’t afford them anymore.  The strategic guidance hit one other very new note: While American forces will continue to maintain a significant presence in the Middle East, the planners wrote, “We will of necessity rebalance towards the Asia-Pacific region.” This is bureaucratic code for “we will stand up to China,” which, the Obama administration has concluded, has superseded Al Qaeda as the chief future threat to American national security.

To say this is not merely to assert that one region has taken precedence over another but that the traditional threat of the expansionist state has supplanted the threat of the stateless actor that emerged after 9/11. Of course, global problems like climate change, epidemic disease, nuclear proliferation and terrorism won’t go away. But in matters of war and peace, we seem to be returning to a more familiar world in which great powers manoeuvre for advantage.

We left that world behind, or so we thought, with the end of the cold war, which deprived America of its traditional enemy and thus raised the question of whether and when we would resort to force.  The answer came in the mid-1990s, when the Clinton administration felt compelled to respond to political chaos in Haiti and mass violence in the Balkans. Force could be used in the pursuit of justice. During the 2000 election campaign, George W. Bush vowed to put an end to these moralistic enterprises and to focus instead on great-power relations.  But 9/11 turned those plans upside down. Indeed, the Bush administration’s 2002 national security strategy asserted that “America is now threatened less by conquering states than we are by failing ones.” Bush, far more than Clinton, yoked the use of force to a transcendent principle, insisting that America “must defend liberty and justice because these principles are right and true for all people everywhere.”

Those were fighting words, and not just abroad. The debate over the war in Iraq revived many of the old debates from the Clinton era. Liberal internationalists like the British prime minister, Tony Blair, joined American neoconservatives like William Kristol and Robert Kagan in arguing for the use of force to bring about transformative political change, while “realists” on the left and right warned of the danger of reckless adventures.

The era we have now entered will be a less ideologically charged one. The questions raised by China’s growing ambitions are categorically different from those provoked by 9/11. China is an emerging power, and once having found their footing, emerging powers usually seek to expand at the expense of their neighbours.

The world is accustomed to dealing with this kind of problem, which involves persuading the bumptious power that its interests lie in cooperation rather than in confrontation. And there is a fair amount of consensus in policy circles about how to deal with it. Conservatives have been sounding alarms about China’s military ambitions for several years, and the Obama administration has now begun to execute a “pivot” to Asia. On a visit to the region, President Obama announced that America would station 2,500 Marines in Australia, even as it decreased military commitments elsewhere.

Whatever, policy the Obama administration or its successor adopts toward China, the broader East Asian region, unlike the Middle East, is filled with stable, and largely democratic, states. The United States does not have to defend liberty and justice there. Regime change, democracy promotion and nation-building will be off the table. So, for that matter, will war.  America is not about to go to war with China, or with anyone else in Asia. The struggle to balance Chinese ambition will be left mostly to the Navy and Air Force, and our allies in the region. And it will not be a metaphysical one: the very complicated relationship with China is much less a clash of worldviews than of interests.

Finally, there is the elemental fact that America can no longer afford its own ambitions. The failure of last year’s bipartisan effort to solve the deficit crisis triggered automatic cuts that are supposed to double the half-trillion dollars already scheduled to be sliced from the Pentagon budget.

In his 2010 book, “The Frugal Superpower,” Michael Mandelbaum argued that the contraction of the American economy meant that “the defining fact of foreign policy in the second decade of the 21st century and beyond will be ‘less.’” Mandelbaum, himself a leading realist, suggested that the chief victim of the new austerity will be “intervention.”

It may be so, though the Nato air campaign in Libya shows that humanitarian intervention is neither defunct nor doomed to failure. Such ventures, however, will be very rare, as the current stalemate over Syria implies. The coming years may well be a period of at least relative austerity, modesty and realism. Should we feel relieved?

It is easy enough to say that the United States should no longer fight wars of occupation in the Middle East, or seek to promote democracy through regime change, or undertake counterinsurgency campaigns on a massive scale. But in a world of weak and failing states, are we also to abandon ambitious hopes to help build stable and democratic institutions abroad? Is foreign aid to wind up on the junk heap of failed dreams?

America has been and can continue to be a force for ‘good’ in the world. But those of us who have championed an idealistic foreign policy have been deeply chastened by the failure of so many fine hopes and have been forced to recognise both how much harm the United States can do with the best of intentions and how very hard it is to shape good outcomes inside other countries. So we must accept, if uneasily, the future which now seems to lie before us: We will do less good in the world, but also less harm.     –NY Times