WASHINGTON -- "We've heard it again and again," Gen. Colin L. Powell has said before. "America cannot be the world's policeman. Yet, as I've learned time and again . . . when somebody needs a cop, guess who gets called to restore peace? We do."
Once more, the chairman of the Joint Chiefs of Staff is using his considerable persuasive powers at the White House to warn against deeper U.S. military involvement in the Balkans.
Right now, as far as is publicly known, U.S. action in Bosnia and Herzegovina is limited to participation in an allied airlift of food, medicine and other necessities to the people of Sarajevo, besieged for four months by Serbian nationalists.
Pentagon officials have professed to believe, optimistically, that it could be kept at that. Now they are having to include likely use of some naval and air units to enforce an embargo and cover any allied ground convoys that may be dispatched.
But experience and Western leaders' escalating warnings to the Serbs indicate that much more strenuous measures can be expected.
Soon, these leaders may have to make good on their threat to use military force to ensure delivery of supplies to besieged Sarajevo and to other points in Bosnia as well.
If so, how far in will they get, and how will they get out? They are at the mercy of the Serbs, who can hide out in the mountains and hold back or loose their artillery at will.
Successful armed intervention, to protect overland relief convoys and shield the people being helped, could eventually call for tens of thousands of troops and a long stay -- depending on the whims of the Serbs.
France and Italy are discussing providing armed convoys. President Bush has not ruled U.S. ground troops in or out, according to Secretary of State James A. Baker III, but Mr. Baker thought their use "quite unlikely." That has been the case before, starting with the Korean War in 1950, when Washington tried at first to get by with air and naval power, and soon followed with ground forces.
The mountainous overland routes in the former Yugoslavia are ideal for ambushes, as legions of outsiders -- notably the Germans in World War II -- have discovered to their sorrow.
Around Sarajevo and its airport, the Serbs hold the high ground. This recalls the predicament of U.S. Marines as would-be peacekeepers at Beirut airport in 1983 and as defenders of the Khe Sanh base beneath Vietnamese mountains during the Tet offensive in 1968.
Retired Marine Lt. Gen. Bernard E. Trainor, now director of Harvard's national security studies program, estimates that 50,000 troops would be needed to secure an area of 20 miles around Sarajevo and its airport in order to keep them out of artillery range, and to keep roads open.
Describing a somewhat similar undertaking to take over mountains and approach routes, the respected German commentator Josef Joffe estimated in the Munich Sueddeutsche Zeitung newspaper that "several brigades" of about 4,000 men each would be needed.
There has been official talk here, and at the Munich and Helsinki summits, of employing U.S. air power as well as naval units in the Bosnian crisis.
Questions arise therefore as to whether this would include strikes against Serbian anti-aircraft weapons in the hills, followed by bombing of artillery emplacements.
Mr. Baker said yesterday that Mr. Bush would consider a Bosnian appeal for attacks on the artillery. It could be a big effort, if undertaken.
Another nightmare scenario suggests itself and explains concerns about long involvement. Suppose the Serbs lie low during the relief of Sarajevo -- as they haven't yet -- and bide their time until the relievers are gone and then start up again.
All historical precedent says that undertakings like those that could now be in train in the Balkans are costly, require large forces and have unpredictable outcomes.
Getting out would be a deal harder than getting in.