What is the right foreign policy for a downsizing superpower?

In the years ahead, the United States will have to take dramatic steps to curb its ever-expanding fiscal deficits. No matter what our politicians promise, this will mean both raising taxes and cutting expenses, including Social Security, Medicare and national defense - the largest items in the federal budget.

In these circumstances, with Americans sending more tax dollars to Washington and getting less in return, they will be less generous in supporting not only defense spending, but also diplomacy, foreign aid and the other tools of U.S. foreign policy. President Obama's budget proposal already begins this process by reducing the Pentagon's spending by $78 billion over the next 10 years, but far deeper cuts are sure to come.

A smaller defense budget and less ambitious international commitments won't necessarily herald the end of America's era as a global superpower. But they do mean that we will have to be much more selective about where and how we deploy our military and diplomatic resources. We will have to distinguish between the military missions, weapons systems and diplomatic initiatives that are vital to the safety and prosperity of the American people and those that are merely desirable - and therefore, in our current fiscal straits, dispensable.

In deciding which foreign-policy priorities to retain and which to discard, the United States should follow a simple, and perhaps counterintuitive, rule: Our older commitments - those carried over from the Cold War - are more important than the new ones we've taken on in the last two decades. Those older commitments include the U.S. security guarantees and military presence in the Middle East, East Asia and Europe; while the newer, expendable commitments involve expensive and protracted nation-building exercises of the kind we've carried out in Somalia, Haiti, Bosnia, Kosovo and, most burdensome of all, in Afghanistan and Iraq.

Continued American influence in the Middle East is crucial, of course, because the region harbors so much of the world's oil. It is also a region where the United States may have to do more, not less, in the years ahead. Egypt, for example, has long been a pillar of American strategy in the Middle East. The uncertainty of the post-Mubarak era means that America's annual $1.6 billion aid package to Egypt should continue and perhaps even increase, because it gives Washington some influence over the institution that will have a major say in the country's future - the Egyptian army.

That said, the most immediate danger to U.S. interests in the region comes not from instability in Egypt but from ambition in Iran. The country's radical regime seeks to dominate the region, which would give it sway over the oil reserves so critical to the world's industrial economies. The other oil-producing countries of the Middle East are not capable of defending themselves against Iran; that task falls to the United States.

The Obama administration has stated its commitment to preventing Iran from obtaining nuclear weapons. Doing so might require a military strike on Iran's program. A war with Iran, like all wars, would be a decidedly perilous undertaking, but this administration, or its successor, may ultimately decide that any alternative policy would be even more perilous.

By contrast, there is no immediate prospect of war in Europe or East Asia. The United States contributes to peace in both by serving as a buffer between and among regional powers that, while not preparing for armed conflict, do not fully trust one another.

In Europe, a U.S. military presence provides reassurance that, should Russia become more aggressive, the United States would be on hand to help deter it, as it did during the Cold War. That same U.S. presence, however, also reassures the Russians that Germany, a 20th century rival, will remain safely anchored in an American-led alliance and will therefore not feel the need, for defensive reasons, to boost its military forces or obtain its own nuclear weapons.

Similarly, in East Asia, U.S. military deployments reassure Japan and other countries that if they have to confront China, the United States will stand with them, while at the same time reassuring the Chinese that Japan - its occupier in the first half of the last century - will not threaten China.

Of course, we cannot be sure what foreign policies China and Russia will adopt. The Chinese insist on the right to govern Taiwan, and they assert sovereign claims to parts of the East and South China Seas that other countries consider illegitimate. But launching military adventures would jeopardize the trade and investment that have fueled the country's economy in recent decades, so Chinese foreign policy will probably remain relatively restrained. Similarly, Russia's leaders have claimed privileges in neighboring countries that were once a part of the Soviet empire. As a declining power, however, Russia probably lacks the capacity to impose its will on its neighbors.