BERLIN -- For the British army, entire books are filled with chronicles of grim, historic retreats. There was the narrow escape from the Nazis at the Arnhem bridgehead 50 years ago, and, still more haunting, the disastrous 18th-century withdrawal through the narrow passes of Afghanistan, just to name two such episodes.
Now some military analysts worry that a new chapter could be about to unfold in the hills of Bosnia, where a few thousand British and French soldiers are the backbone of 24,000 United Nations peacekeeping troops from 17 countries.
With talk of a possible pullout accelerating after a defiant Serbian attack against the Bosnian Muslim enclave of Bihac, troubling questions abound: How quickly could a withdrawal be completed? By what means? How much equipment should be taken, or abandoned? And most important: How dangerous would it be?
"The problem is, the moment it is known the U.N. is leaving, there will be a flare-up of fighting all over Bosnia, because that will be the moment everyone starts scrambling for position," said Jonathan Ayal, a military analyst for the Royal United Service Institute in London. "You may then end up in a situation where you have to fight your way out. This is everybody's nightmare. There will be people killed, and they will be killed for nothing."
Already it is clear among U.N. military planners that the difficulty of withdrawal had long been underestimated. Until the Bihac debacle, the misplaced hope was that the war would either stabilize or end, allowing for an orderly, even victorious, withdrawal.
Now, according to U.N. forces spokesman Alexander Ivanko in Zagreb, Croatia, "the plans are being updated on a regular basis," although he cautioned, "Right now it is still only hypothetical planning."
Most plans call for a drawn-out operation that would last several months -- as long as 150 days, according to U.N. estimates. This would involve extensive airlifts and several road convoys.
Lately the United Nations has been unable to travel through either method in much of Bosnia. The Serbs have kept Sarajevo's airport closed by setting up a battery of anti-aircraft missiles within range, while in other parts of the country Serbian soldiers have halted convoys of supply trucks.
The ease with which the convoys have been stopped foreshadows the possible hazards of carrying out any of the evacuation by road. Most of the Bosnian countryside is wooded mountains, with almost every curve in the road offering a fine vantage point for an ambush.
There are also two wild cards that could haunt the operation from the outset, U.N. officials say.
The first is how desperate Bosnia's majority Muslim population might become, especially in the surrounded Muslim enclaves of Sarajevo, Gorazde, Srebrenica, Zepa and, now, Bihac.
There would be nothing to prevent people desperate for food from lying down in front of U.N. vehicles to prevent their departure.
If the military protection of aid convoys were withdrawn, U.N. aid workers would also pull out, leaving tens of thousands of civilians facing starvation.
"What that leads to is women and children saying, 'We don't want you to go. If you want to pass this spot, it will be over our bodies,' " said a NATO officer, who spoke on condition of anonymity.
Human roadblocks would be likely to occur along any evacuation route, he said.
The second wild card is what will happen to the heavier weapons and equipment of the U.N. forces. A Nordic battalion near Tuzla, in a region held by the Bosnian government's mostly Muslim forces, has four light tanks. Scattered elsewhere in the country are hundreds of armored personnel carriers, dozens of which are armed with heavy anti-aircraft cannon.
Bosnia's weapons-hungry warring factions would be expected to try to seize that armament. If the evacuation plan called for destruction of the weapons, the local forces would hardly stand by and watch the process.
NATO planners believe that their forces might also become the target of pre-emptive strikes, attacks that the local forces could launch to capture the strategically placed international observer compounds.
"The peacekeepers are there as peacekeepers, and are armed as peacekeepers. They are not in a good position to fight their way out by themselves," said a NATO officer familiar with the contingency planning for an evacuation. "It's a real nightmare."
If all these worst-case scenarios come true, U.N. forces might find themselves having to drop their equipment and escape by air as quickly as possible, with heavy air support from NATO jets.
"If they had to, they could evacuate the personnel helicopter-style, sort of like Saigon, '74-'75," said Mr. Ayal, referring to the final days of the U.S. departure from South Vietnam. Panic-stricken Vietnamese clung to the struts of U.S. helicopters that lifted off from rooftops as the city fell.
"The enduring images of something like that would be very costly for Europe," Mr. Ayal said.