By Matthew Davis
BBC News, Washington
The US strategy on the ground in Iraq is to "clear, hold and build"
Another death, another grim milestone for US forces in Iraq.
First it was 100, then 1,000, now 2,000 killed in pursuit of a goal still elusive two-and-a-half years after the invasion.
Whether a landmark or - as the Pentagon says - "an artificial mark on the wall", the latest casualty count is a reminder of the hard road still ahead in Iraq - and at home.
It was supposed to be so easy.
The same images are still fresh in the mind: the triumphant toppling of Saddam's statue, President George W Bush flying in to declare "Mission accomplished", a defiant president urging soldiers, "Bring them on".
But now flagging public support for the mission, and the president, is facing another psychological test.
War critics are using the occasion to advance their cause.
Candlelight vigils are planned across the states. Activists will besiege the gates of the White House. Television ads will pose the question, "How many more?"
In anticipation of the barrage of criticism, the White House is trying to put the losses into perspective, talking up the success of Iraq's constitutional referendum - and standing firm on the mission.
On Tuesday, Mr Bush again portrayed the conflict in Iraq as the central front of the war on terror, saying winning it was the best way to stop terrorists striking America again.
The troops deserved "unwavering commitment to the mission and a clear strategy for victory", he told a luncheon for military spouses at a US air base.
But he also warned that progress would require "more sacrifice, more time and more resolve".
Cold military logic dictates that current casualty rates pose little threat to the viability of US operations in Iraq.
The Pentagon says the 2,000th death in Iraq is an "artificial" mark
The losses are tragic, but sustainable, and do not compare to the 58,000 US deaths in Vietnam, or the 37,000 troops killed in Korea.
Officials point to the relatively low level of violence in the three-day lockdown of Iraq during the constitutional referendum as a positive sign.
It is hoped that political progress will take its own toll on the power and pull of the insurgency. Yet the bombings continue and US casualty rates show little sign of slowing.
Military officials have concerns about new trends in the improvised explosive devices (IEDs) used by insurgents.
US forces have now seen repeated uses of "explosively formed projectiles" able to pierce through armoured vehicles, posing a more deadly threat to soldiers.
Meanwhile, a leading military think tank said continuing violence and instability was likely to mean US troops would probably have to remain in Iraq until well after the US presidential elections in 2008.
"We're likely to see continued bloodshed and instability inside Iraq," said Patrick Cronin, director of studies at the International Institute for Strategic Studies.
"This is a long-term proposition, and I would expect the next US administration to have forces inside Iraq at a fairly large number for some years to come."
There are now about 159,000 US troops in Iraq. While 2,000 soldiers have died since March 2003, a further 15,000 have been wounded.
Anti-war protesters are expected to congregate at the White House
Such sacrifices are taking a toll on public opinion.
For the first time, a majority of Americans believe that the Iraq war was the "wrong thing to do", according to a new poll in the Wall Street journal newspaper.
Some 53% of those surveyed said they felt taking military action against Iraq was the wrong move, against 34% who thought it was correct.
Meanwhile, Mr Bush's personal popularity is also at an all-time low.
The commander-in-chief is standing firm on the Iraq mission, but events may send the political costs of the war even higher.
The most immediate threat is the culmination of the investigation into the leaking of a CIA agent's identity, the central issue of which harks back to the justifications used for the Iraq war.
If anyone in the administration is indicted it will revive an uncomfortable debate.
Measures of progress on the ground are fast needed to balance the count of lost lives that for some has again become a gauge of the president's wisdom in deciding to invade.