WASHINGTON, Nov. 9— The capture of Mazar-i-Sharif delivers the first tangible victory for the American-led campaign after more than a month of war.

If the Northern Alliance maintains control of the strategically important city it will cut off Taliban forces to the east. It will open a land corridor for relief aid for millions of hungry Afghans. And it will furnish the American military with a potential staging ground for military operations inside the country.

It will also provide a badly needed public relations boost for a military operation that has been long on airstrikes -- more than 8,000 bombs have been dropped -- but devoid of gains on the ground.

''I don't think there is any doubt at all that the military momentum is now moving against the Taliban,'' British Prime Minister Tony Blair said today, eager to exploit the first bit of good news.

Still, for all of the applause in Washington and London and the very substantial setback the loss of the city represents for the Taliban, its conquest is not a knockout blow.

Located on the steppe in the northern part of Afghanistan, Mazar-i-Sharif is far from Kandahar, the Taliban's political base in the south. To prevail in the conflict, the American military will have to find allies among the Pashtun tribes in the south, help equip and arm them and provide them with air cover so that they also can take on the Taliban.

And the American military will have to find a way to get its Special Operations forces, which have not carried out a raid since Oct. 19, back into the fight. In the end, the United States may even have to dispatch a significant number of ground troops, an option that the Pentagon has not ruled out but which the Bush administration is clearly hoping to avoid.

The successful attack on Mazar-i-Sharif was a direct result of the Bush administration's decision to aid the Northern Alliance. The city, in fact, was a target from the opening days of the American military campaign, when a garrison of Arab and other foreign fighters recruited by Osama bin Laden was bombed.

But in recent days the American military has made a concerted effort to bomb the Taliban forces that were defending the city. American commandos near Mazar-i-Sharif, in turn, called in the airstrikes and helped plan the attack on the city itself. The American military even delivered food for the Northern Alliance's horses.

The attack on the city involved three groups of Northern Alliance forces: fighters led by Gen. Abdul Rashid Dostum, the ethnic Uzbek warlord; fighters led by Cmdr. Ostad Atta Muhammad, an ethnic Tajik, and fighters led by Cmdr. Haji Mohaqiq, who is an ethnic Hazari.

The Taliban fled the city, apparently fearing they would be killed in a wave of reprisals for thousands of killings the Taliban carried out when they took the city. American air power should ensure that the city remains in the hands of the Northern Alliance.

The capture of Mazar-i-Sharif could lead to the rout of Taliban forces in the northern tier of the country. Taliban forces in Kondoz and Taloqan will find themselves cut off from supplies. Taliban forces in the neighboring provinces of Samangan, Sar-i-Pul and Faryab will also be vulnerable.

After consolidating a victory in Mazar-i-Sharif, the Northern Alliance plans to advance toward Herat in the west and the capital of Kabul to the south under the protective cover of American air attacks, said Haron Amin, a special envoy in Washington for the Northern Alliance.

The seizure of the city is also expected to help improve the flow of international aid. Aid can be trucked south from Uzbekistan to Mazar-i-Sharif, which could become an important hub for distributing food and relief supplies. Pentagon officials said the first preference is to have the Northern Alliance or non-American members like the Turks guard the ''land corridor'' from Uzbekistan. But they did not exclude that United States Marines might be brought in from their ships in the Arabian sea to provide security.

Kenneth H. Bacon, the president of Refugees International and the former Pentagon spokesman, said that 75 percent of the Afghan people afflicted by famine live in the northern half of the country. ''From a humanitarian standpoint this is a huge development,'' Mr. Bacon said.

Mazar-i-Sharif also has a large airfield, which along with other airfields the Northern Alliance may take, could become a staging area for American forces inside Afghanistan. ''Having access to an airfield inside the country allows for new freedom of movement,'' Rear Adm. John Stufflebeem, the spokesman for the Joint Chiefs of Staff, said today.

The broader hope is that the seizure of the city will persuade many Afghans to conclude that the days of the Taliban are numbered and encourage them to turn against the regime.

But the capture of Mazar-i-Sharif could turn into a liability if the Northern Alliance fighters carry out a wave of revenge killings. That could lead Pashtuns in the south to rally around the Taliban.

Northern Alliance representatives said they were taking steps to discourage such killings. And there is a plan under consideration to help the Northern Alliance and the Americans make the case that the capture of Mazar-i-Sharif is part of a campaign against the Taliban regime but not the Pashtuns themselves.

The idea is to convene a broad coalition of anti-Taliban leaders from the Northern Alliance and the Pashtuns to discuss how to govern Afghanistan after the Taliban are toppled. One possible venue for the meeting is Mazar-i-Sharif.