WASHINGTON — WASHINGTON -- The U.S. command hopes to use the armor and infantry units now reinforcing the Somalian capital to evict heavily armed Somalian militia forces that have surrounded allied bases in downtown Mogadishu. The militia members, loyal to Gen. Mohammed Farah Aidid, also control all the main roads. In the past week, they have used their positions to kill and wound American soldiers by laying mines in the roads and shelling the bases with mortars. If the mission succeeds, Pentagon officials said, the allies will no longer be pinned down in this way, and key points throughout Mogadishu's downtown will be dotted with garrisons and monitored with patrols and checkpoints. The Americans would then hand over control to United Nations forces from other countries as quickly as possible, and withdraw no later than March 31. It is a job that military planners appear confident of accomplishing if there is little or no armed resistance, but that could prove costly if it must be put into effect by force. Although the military would be happy if President Clinton's emissary to the region, Robert Oakley, could arrange some kind of truce, even one that implicitly protected General Aidid from arrest, they would not welcome a cease-fire in defense of the status quo. To accept that would be to ignore Mr. Clinton's explicit new orders to secure the bases and main roads of Mogadishu, and it would forgo any military option to rescue a wounded American captive, Chief Warrant Officer Michael Durant, a helicopter pilot. Even if he is released, the evolving military plan calls for clearing whole neighborhoods of armed members of the Aidid faction, who otherwise would be able to resume at any time they wished their standard hit-and-run tactics, using mines, mortars, rocket-propelled grenades and anti-aircraft guns. Again and again last week, military officials said they were counting on diplomatic progress toward stabilizing the city so that they could effectively carry out Mr. Clinton's orders. "If you focus only on the military strategy -- only on the military strategy -- clearly this is not a force level to win with," one senior Pentagon official said Friday. Even when there are 32 helicopters, more than 100 tanks and other armor, and 3,000 troops stationed ashore, the military's main task will be to defend a few bases and escort convoys on a few arteries through town. Although thousands of Marines and dozens of strike airplanes will be available just offshore, the Pentagon hopes it won't have to call on them except to rescue units under attack or to retaliate for provocations. The basic task, carried out on the streets of Mogadishu, block by block, day and night, would be left mainly to the Army with its tanks and armored infantry vehicles. One official at the Pentagon's Central Command in Florida, which is in charge of the operation, cited the roads from the airport northward to the university and the U.S. Embassy compound, which is occupied by American troops, and from there on to the logistical bases called Hunter Base and Sword Base, as examples of arteries that must be cleared. In some places, the roads are bordered by walled compounds; in other places by commercial buildings. Sometimes, the roads go through wide open spaces. All along these routes, this official said, the military will try to move back the armed militia and to establish roadblocks and checkpoints, patrolling the roads at night to keep sappers from laying explosives. The trick is to do this without inviting what a senior official called "mission creep" -- the expansion of the role to include, for example, raiding neighborhoods controlled by General Aidid and searching for weapons. The military planning continued this weekend at the U.S. Central Command in Florida, which will control the operation. A new commander of the American task force in Somalia, Brig. Gen. Carl Ernst, has been selected; he will report to Maj. Gen. Thomas Montgomery, who in turn reports to Lt. Gen. Joseph Hoar of the Central Command. General Montgomery is simultaneously the deputy commander of all U.N. forces in the country, under Lt. Gen. Cevik Bir, of Turkey.