The five-month war in Afghanistan has become the U.S. military's longest sustained engagement since Vietnam, with an elusive enemy sheltered in strange and savage terrain.

Still, few Americans have died in combat. Although the current Operation Anaconda is an exception, most of the fighting has been conducted with precision weapons dropped by airplanes and small special forces units working with Afghan militias.

No war will be exactly like that in Afghanistan, analysts and military officials say. But the battles have had many features that probably will be repeated in the war on terrorism and in future conflicts.

"You really have a trend there," says military analyst Michael Vickers of the Center for Strategic and Budgetary Assessments. "We're fighting asymmetrically, using areas where we can hit the enemy and they can't hit us."

So far, 13 Americans have been killed in combat in Afghanistan. The United States has about 5,300 soldiers in the country and about 60,000 military members in the region supporting the effort.

By comparison, during the six-week Gulf War in 1991 that pushed Iraqi forces out of Kuwait, there were 147 American combat deaths. More than 500,000 U.S. troops took part in the war.

The Vietnam War lasted the better part of a decade, with more than 47,000 soldiers killed in combat.

Pentagon officials will not estimate how many Taliban and Al Qaeda fighters have died in the war, though the number probably is at least in the thousands. Several hundred have been killed in the latest operation alone, military officials have estimated.

An additional 525 fighters are U.S. detainees, with 225 in Afghanistan and 300 at the U.S. Navy base in Guantanamo Bay, Cuba.

Both the Gulf War and the Afghan conflict started with massive bombing campaigns. Unlike the Gulf War, most of the bombs dropped on Afghanistan have been guided to their targets by lasers or satellites.

Guided weapons mean pilots can fly fewer missions, drop fewer bombs, put fewer U.S. troops at risk, kill fewer civilians -- and more effectively destroy enemy facilities.

When it came to ground combat, the Gulf War was a more traditional clash between two nations' military forces, played out in the expansive deserts of Kuwait and Iraq.

In Afghanistan, American forces have helped anti-Taliban forces oust the Islamic militia from power and pursue Al Qaeda terrorists.

But Afghanistan has no army or police force to keep order once the United States leaves. The Pentagon plans to help train and equip an Afghan army. Experts say they expect the U.S. military to remain in Afghanistan at least through this summer.

"We are going to be engaged in this for many months to come," said analyst John Pike of GlobalSecurity.org.

Since the Taliban's collapse in December, most of the fighting has been with small, isolated groups of Al Qaeda and Taliban members, many of whom are hunkered down in well fortified caves or tunnels.

"It's been kind of like whack-a-mole: Whenever someone pops their head up, we go in and kill them," said Vickers, a former military and CIA special forces operative.

Operation Anaconda is an exception. More than 2,000 U.S., Afghan and allied troops are encircling hundreds of Al Qaeda fighters in 60 square miles of rugged, snowy mountains.

Wiping out small numbers of fighters hiding in formidable terrain is difficult, which is why Rumsfeld and other defense officials are quick to say U.S. troops will not soon be leaving Afghanistan.