Strange, isn't it? United Nations Secretary-General Kofi Annan urged caution when it came to intervening in Iraq when it was ruled by a regime that posed a grave and gathering threat to U.S. security. Now he wants the United States to commit troops to Liberia, a place where no U.S. interest is at stake.

That's not to say the situation in Liberia isn't serious or tragic. More than 100 people have died there over the last few days, and they represent only the latest casualties in that nation's civil war. And it's not to say we should do nothing to help the people of Liberia. But sending U.S. troops to act as peacekeepers would be a mistake.

For one thing, no outside troops will be much good in Liberia now. Civil wars must be solved initially from within. Peacekeeping outsiders can't stabilize an inherently unstable situation. And it's a particularly bad idea to call on the U.S. for a Liberia force.

Like it or not, people fight. They do so often and for a variety of reasons, and it's unreasonable -- even impossible -- to expect the U.S. to play global cop on the beat. More important, it would jeopardize the work this country does every day to make the world a safer place.

Consider the unique role the U.S. plays in international relations. One might argue that it has the most important peacekeeping role in world history: keeping the big peace. It must concentrate its power not on battles that can be solved internally, country by country, but on problems that spill across borders and threaten regions and global security.

The U.S. does this by ending major threats, deterring large-scale aggression and maintaining order on the high seas, missions that are not only for the global good but also protect vital national interests of the U.S. and its allies.

When global threats emerge, from international terrorism to Saddam Hussein, only the U.S. can respond quickly and restore order. No one likes war, and throwing military resources at a crisis is not always the answer; indeed, it is rarely the answer. But only the U.S. can conduct large-scale expeditionary warfare when it is needed to address major threats and bring peace.

There will always be leaders in the world willing to amass power by force unless there's something to stop them. Today, that something is the U.S. and its system of alliances. Without the U.S. making a significant commitment in troops, money and other resources to its regional alliances, there would be no credible deterrent to stop the next Hitler or Stalin.

Ever notice how global trade continues over the oceans without much trouble? This wasn't always the case. Criminals would choose to disrupt this global access, and states would sponsor blockades. But today the U.S. maintains a blue-water navy no nation could consider challenging, which ensures open access to the world's oceans.

Unfortunately, the U.S. can't sustain such vital missions and at the same time undertake cop-on-the-beat peacekeeping.

Peacekeeping diverts considerable resources away from a fighting force. In the Balkans, for example, we have 8,000 troops deployed as peacekeepers, and three times that when it comes to the real commitment -- for every soldier in the field, one is preparing to deploy and one is recovering and often retraining for war fighting. Similarly, if the U.S. were to put 2,000 peacekeepers in Liberia per Annan's request, a total of 6,000 would be required, which would mean 30,000 troops committed to missions that others could handle.

Peacekeeping also stretches thin many of the same specialties crucial to the war on terrorism. Both tasks place a premium on special operations units -- for example, reconnaissance assets and military police units.

And then there's the question of money. Peacekeeping drains billions of dollars from the Defense budget. In the 1990s, U.S. peacekeeping efforts in Somalia cost $1.5 billion, in Haiti, $1 billion, and the continuing Balkans operation has cost $20 billion so far.

So should we do nothing in the face of civil wars or country-by-country hot spots? No. The U.S. does have a role to play in places like Liberia. We might help arrange an international peacekeeping effort, assuming the conditions are right for success. We could provide logistics support and communications capabilities. A few high-ranking officers could be committed to run the operation if the international community needs such help. Indeed, these may be legitimate roles for the Marines that President Bush ordered to Liberian coastal waters Friday.

There is a lot of peace to be kept in the world today. Peace-loving nations, working together, can accomplish a lot. But asking one nation to do it all will result in nothing being done properly. Other nations must take responsibility for some of these other operations -- so that the U.S. can keep the big peace.

Jack Spencer is a senior policy analyst for defense and national security in the Davis Institute for International Studies at the Heritage Foundation.