The involvement of US Marines in Afghanistan may, in retrospect, turn out to be a watershed moment in the war on terrorism.
But for now, significant numbers of US troops on the ground raise important questions about how the war is to proceed.
Will things continue to crumble rapidly for the Taliban, or is there more likely to be a lengthy and dangerous period of ferreting out determined Al Qaeda and Taliban "foreigners"?
At what point can a military "victory" (however one defines that) be declared?
Perhaps most sensitive, how much is the public willing to accept American casualties, which President Bush now indicates are inevitable?
The quick seizing of ground by US troops is complicating political issues as well. The US has to avoid appearing like an occupation force, while searching for a way to expedite formation of a post-Taliban government that is stable and representative enough to take control from US troops.
The insertion of the Marines "is clearly a commitment of the United States to the effort, and it's a risk in terms of the casualties that can be incurred," says Scott Gartner, a political scientist at the University of California at Davis who specializes in military affairs. "In that sense, it's clearly a change."
Experts say the next phase in the war, tracking down Al Qaeda and Taliban forces, will be far more difficult than attacking them from the air with relative impunity as US forces have been doing.
"Between hiding out and blending in, the remnants may be quite difficult to mop up," says Larry Seaquist, a retired US Navy warship captain and Pentagon strategist. "The extreme, puritanical message of the Taliban and Osama bin Laden will not necessarily be dirtied by their military setback in Afghanistan. Indeed, without a country to run they may be more on message than ever."
Afghan forces, too, are notorious for their "long memory." "Part of the Pashtun Wali, or code of honor, allows - no expects - that honor debts will be avenged and that such obligations can be passed on from generation to generation," says a former US Army officer who has spent much time in Afghanistan. "This obligation will extend across national boundaries."
The US obligation, as many see it, is to confront those guerrilla forces directly, as risky as that might be.
So far, only a small number of US troops have been killed or injured in activities related to the war - most recently in a "friendly fire" incident in which five service members directing a close-air support strike were seriously injured in a bombing raid.
"Obviously, no president or commander in chief hopes anybody loses life in the theater, but it's going to happen," President Bush said Monday.
Despite the difficulties ahead, most experts say a significant presence of American ground troops is essential to the mission.
"It was necessary not only from an operational perspective, but necessary from a psychological and political perspective," says Daniel Goure, a military analyst with the Lexington Institute in Arlington, Va. "This could not be America's 'war on terrorism' fought by surrogates. We had to be on the ground."
Beyond enemy forces that must be killed or captured (if they don't surrender), it will be important for US forces to gather intelligence to prevent future terrorist attacks. That includes scouring through paperwork left behind in safehouses and caves. "All that has to be done before we can declare victory," Dr. Goure says.
Rhetorically, at least, the presence of Marine forces in Afghanistan adds weight to the Bush administration's vow to fight terrorism in other countries as well. As marines were fighting their first battles, the president was warning Iraq's Saddam Hussein that he "would be held accountable" if he were seen to have supported terrorists or developed weapons of mass destruction.
"Had the president made those kinds of threatening gestures to Iraq prior to the insertion of marines, it would have seemed like cheap talk," says Mr. Gartner. "But with significant Marine troops being deployed, and that being a costly signal of United States commitment to the war against terrorism, then the threatening words to Iraq have a different meaning today than they would have four or five days ago."
All along, most observers - including the president and Defense Secretary Donald Rumsfeld - have agreed that the fight against international terrorism will be a long and difficult one.
"In this war, victory is a function of vigilance," says the former Army officer. "We must take a very long view, and the nation and the press should stop expecting something quick or easy."
Especially, others add, if US ground forces get bogged down in Afghanistan while political factions there continue their historic and sometimes violent competition.
Says former Navy Captain Seaquist: "If the new government looks like the same old thugs doing the same old wrangling, and we look like - or are being portrayed as - yet another occupying power, we could see our image flip from savior to invader."