LOS ANGELES – As the United States stumbles through its economic challenges at home, the pressure of world events will not subside. But America’s ability to address them has changed. Its fiscal weakness limits its ability to act as global policeman. Despite the relatively costless overthrow of the Qaddafi regime, America’s prolonged interventions in Afghanistan and Iraq have severely strained the public’s tolerance for an active foreign policy.

Nonetheless, the US seems destined to remain the world’s most important actor for the foreseeable future. But today it is an actor without a script – it lacks a strategic guide comparable to the Cold War’s containment doctrine to prioritize policy.

Quite simply, the ad hoc policymaking that directed interventions in the Balkans, Somalia, southwest Asia, and the Middle East in the past two decades will not suffice in this new era of limitations. This suggests that the US should seek out an overarching strategy to discipline its impulses to fight wars of choice or engage in nation-building efforts.

President Barack Obama’s 2010 National Security Strategy nurtures broad policy aspirations – "[n]ow we must position the United States to champion mutual interests among nations and peoples” – but falls short as a practical guide. I suggest an alternative strategy, one already embedded in America history, though largely unrecognized. But making explicit what lies implicit can sharpen US decision-making.

I call this strategy the "Watershed Doctrine.” A watershed is a tipping point, a turning point, a game changer. When the US has confronted a "negative watershed” – a lethal threat to the country – it committed meaningful financial and human resources in order to address the risks. Positive watersheds – opportunities to engineer seismic shifts in international or regional political affairs through nation-building, or to use economic and military assistance to prevent plausible negative watersheds – demand an equal level of commitment.

The Watershed concept provides policymakers with a standard to use – or at the very least to debate. It is an organizing tool for policymaking: Is an international challenge a watershed or not? If so, get involved. If not, stay out.

We find watersheds throughout American history. The War of 1812 and the Civil War are clear examples. Had American forces not expelled the British from US territory in the first, and had Abraham Lincoln and the Union not prevailed in the second, the country would have been balkanized and unable to become the dominant power of the twentieth century.

By contrast, America’s flirtation with colonialism in the Spanish American War, its involvement in Mexico, Central America, and the Caribbean throughout the twentieth century, and, arguably, World War I, were not watersheds for the US. But America’s inability after the Great War to overcome Old World politics at Versailles and isolationism at home marked a failed opportunity to promote a positive watershed.

That failure placed the world on the path to the negative watershed posed by Nazi Germany and Imperial Japan. Nothing foreordained that the US and its allies would prevail. Had the Axis’s negative watershed succeeded, the US would have become a far different country.

A positive watershed, under-appreciated today, developed in the years immediately after World War II with the political transformation of Germany and Japan. America’s remarkable investment of resources in this outcome made both countries stable, peaceful democracies, thereby eliminating them as adversaries and turning them into vital bulwarks against the next harbinger of a negative watershed, the Soviet Union.

Unlike the battle against the Axis, the US fought the Cold War in many ways, on many fronts, and over many decades – using politics, economics, and nuclear deterrence, as well as limited armed action, to ensure the USSR’s containment. In time, the US had to accept that each political contest or military battle lost was not a watershed as long as its core interests in Europe, the Far East, and Latin America were not threatened. Through trial and error – backed by a durable political and economic system – the US prevailed and the Soviet Union disintegrated.

The rise of Islamic fundamentalism poses another historic challenge, though one that is far more inchoate than any that the US has faced before. In other times, the challenge would not even be called a watershed. But the risk that weapons of mass destruction could be turned against the US makes it so. Then there is the "Arab Spring,” a potential positive watershed that calls upon the US to decide how deep a political, economic, and military commitment it ought to make to nurture positive results.

Today, the US is a more sober and realistic country than it was in the heyday of the early post-Cold War period. But, in the aftermath of setbacks in regions where it intervened, and with heightened economic distress at home, the US finds itself uncertain about how to respond to changing global events. Pursuing a "Watershed Doctrine” might provide the right answer.

Bennett Ramberg served in the Bureau of Politico-Military Affairs in the George H.W. Bush Administration. He is the author of several books on international security.

Copyright: Project Syndicate, 2011.