Robert Kagan, a senior fellow at the Brookings Institution, writes a monthly column for The Post. His latest book is “The World America Made.”

These days “soft” power and “smart” power are in vogue (who wants to make the case for “dumb” power?) while American “hard” power is on the chopping block. This is, in part, a symbolic sacrifice to the fiscal crisis — even though the looming defense cuts are a drop in the bucket compared with the ballooning entitlement spending that is not being cut. And partly this is the Obama administration’s election-year strategy of playing to a presumably war-weary nation.

But there is a theory behind all this: The United States has relied too much on hard power for too long, and to be truly effective in a complex, modern world, the United States needs to emphasize other tools. It must be an attractive power, capable of persuading rather than compelling. It must convene and corral both partners and non-partners, using economic, diplomatic and other means to “leverage” American influence.

These are sensible arguments. Power takes many forms, and it’s smart to make use of all of them. But there is a danger in taking this wisdom too far and forgetting just how important U.S. military power has been in building and sustaining the present liberal international order.

That order has rested significantly on the U.S. ability to provide security in parts of the world, such as Europe and Asia, that had known endless cycles of warfare before the arrival of the United States. The world’s free-trade, free-market economy has depended on America’s ability to keep trade routes open, even during times of conflict. And the remarkably wide spread of democracy around the world owes something to America’s ability to provide support to democratic forces under siege and to protect peoples from dictators such as Moammar Gaddafi and Slobodan Milosevic. Some find it absurd that the United States should have a larger military than the next 10 nations combined. But that gap in military power has probably been the greatest factor in upholding an international system that, in historical terms, is unique — and uniquely beneficial to Americans.

Nor should we forget that this power is part of what makes America attractive to many other nations. The world has not always loved America. During the era of Vietnam and Watergate and the ugly last stand of segregationists, America was often hated. But nations that relied on the United States for security from threatening neighbors tended to overlook the country’s flaws. In the 1960s, millions of young Europeans took to the streets to protest American “imperialism,” while their governments worked to ensure that the alliance with the United States held firm.

Soft power, meanwhile, has its limits. No U.S. president has enjoyed more international popularity than Woodrow Wilson did when he traveled to Paris to negotiate the treaty ending World War I. He was a hero to the world, but he found his ability to shape the peace, and to establish the new League of Nations, severely limited, in no small part by his countrymen’s refusal to commit U.S. military power to the defense of the peace. John F. Kennedy, another globally admired president, found his popularity of no use in his confrontations with Nikita Khrushchev, who, by Kennedy’s own admission, “beat the hell out of me” and who may have been convinced by his perception of Kennedy’s weakness that the United States would tolerate his placing Soviet missiles in Cuba.

The international system is not static. It responds quickly to fluctuations in power. If the United States were to cut too deeply into its ability to project military power, other nations could be counted on to respond accordingly. Those nations whose power rises in relative terms would display expanding ambitions commensurate with their new clout in the international system. They would, as in the past, demand particular spheres of influence. Those whose power declined in relative terms, like the United States, would have little choice but to cede some influence in those areas. Thus China would lay claim to its sphere of influence in Asia, Russia in eastern Europe and the Caucasus. And, as in the past, these burgeoning great-power claims would overlap and conflict: India and China claim the same sphere in the Indian Ocean; Russia and Europe have overlapping spheres in the region between the Black Sea and the Baltic. Without the United States to suppress and contain these conflicting ambitions, there would have to be complex adjustments to establish a new balance. Some of these adjustments could be made through diplomacy, as they were sometimes in the past. Other adjustments might be made through war or the threat of war, as also happened in the past.

The biggest illusion is to imagine that as American power declines, the world stays the same.

What has been true since the time of Rome remains true today: There can be no world order without power to preserve it, to shape its norms, uphold its institutions, defend the sinews of its economic system and keep the peace. Military power can be abused, wielded unwisely and ineffectively. It can be deployed to answer problems that it cannot answer or that have no answer. But it is also essential. No nation or group of nations that renounced power could expect to maintain any kind of world order. If the United States begins to look like a less reliable defender of the present order, that order will begin to unravel. People might indeed find Americans very attractive in this weaker state, but if the United States cannot help them when and where they need help the most, they will make other arrangements.