In the wake of its first obvious ground attacks in Afghanistan, the United States faces two new major challenges, one military and one political.
The first is the perception that the insertion of US ground forces - however limited - will be seen as yet another foreign invasion of a country that historically has overcome its ethnic and tribal fractiousness to unite in opposing invasions.
"There has to be a ground component. Nothing else will demonstrate American will and resolve," says a former US Army officer who has spent much time in Afghanistan. "But if there's all this US military activity and presence, it's not going to be seen as an Afghan victory, but rather as something imposed on the Afghan people."
More immediately, putting "boots on the ground" increases the danger that something could go wrong in this conflict halfway around the world. Until the weekend, the war had involved only stand-off air attacks on Taliban and Al Qaeda targets. US fighters, bombers, and cruise missiles have ruled the skies over Afghanistan since the US attacks began two weeks ago, and the potential for loss of American airmen was slight.
Using ground troops is far more dangerous, especially the kind of light infantry and commando operations that began Friday night. It's the riskiest kind of fighting - more susceptible to a failure in mission and more likely to lead to American casualties.
Despite overwhelming US military power, including Special Operations Forces that are far better equipped and more experienced than they were just 10 years ago during the Gulf War, Defense Department officials are under no illusions about how long or how hard the effort will be.
"It's going to be a tough fight," says Gen. Richard Myers, chairman of the Joint Chiefs of Staff. General Myers calls it "the most important tasking the US military has been handed since the Second World War."
Superficially, at least, the war in Afghanistan from the US perspective seems to be a Gulf War in miniature: sustained air attacks, followed by ground forces. But there the similarity ends.
The war in the Gulf involved upwards of 500,000 US troops working from a large base of operations and including tanks and other heavy armor that rolled north toward Baghdad. This time, ground forces are likely to remain smaller in number and be restricted largely to Army Rangers and other special-forces units - including the super-secret Delta Force trained specifically to kidnap enemy leaders.
But despite the smaller numbers, US special forces are seen as the most effective fighting force of their type in the world. They're equipped with precise global-positioning systems, advanced night-vision devices, weapons with laser sights and thermal-imaging capabilities, and computerized fire-control systems.
They are typically backed up by attack helicopters and slow-moving propeller aircraft able to deliver withering cannon fire. For the first time, the US also is able to launch ground-attack weapons from small, pilotless aircraft.
The initial goals of such attacks are to capture intelligence data (and possibly Taliban and Al Qaeda leaders), demonstrate clearly that the US has the capability and the will to put soldiers in harm's way, and force enemy forces to fight on US terms.
"Ground action would have the effect of pulling the Taliban forces back together into fighting groups, which would then become very vulnerable to air attack," says Larry Seaquist, a retired US Navy warship captain and Pentagon strategist.
US ground forces also are meant to bolster rebel groups fighting the Taliban regime. Military officials on the northern Mazar-e Sharif front line, contacted by phone yesterday, confirmed that more than 15 Americans are helping coordinate US airstrikes with Northern Alliance troops against the Taliban.
The Americans are dressed as civilians, but their demeanor signals that "it's clear they have a lot of authority," says Qadratullah Hurmat, a senior aide of alliance Gen. Ustad Atah. "The Americans are important because of their air support," Mr. Hurmat says. "We need their airstrikes to be coordinated with our ground troops."
But beyond the immediate war on the ground and in the air (where US strikes are continuing), there remain longer-range problems. One is the rugged desert and mountainous terrain as well as the coming harsh winter weather. Pentagon officials say US forces are prepared to work in such an environment, even though it makes it far more difficult to root out Taliban troops - whether they're operating from caves or among civilians.
Continued US attacks - especially if they go on for months - also could make it harder to hold together allies and to eventually form a post-Taliban government. The fragile coalition - or "coalition of coalitions," as US Defense Secretary Donald Rumsfeld has called it - could begin unraveling, especially in South Asia and the Middle East if the US-led "war on terrorism" is seen as aimed primarily at Islam.
One officer says privately that the US is beginning the nation-building phase without a very good understanding of those groups opposing the Taliban or who might emerge as the most likely leader.
Staff writer Scott Peterson contributed to this report from Afghanistan.