The US public's souring mood over the war in Iraq is something US military leaders have seen before and learned to dread. In Vietnam, it foreshadowed a humiliating defeat.
Steadily mounting casualties, anti-war protests, crumbling public support and the open political warfare that erupted this week in Washington over Iraq have only heightened the sense of deja vu.
"This is following a political trajectory very similar to Vietnam," said Loren Thompson, a military analyst with the Lexington Institute, a Washington think tank.
"What happened in Vietnam was that as key legislators began to fall away from the president's agenda, military officers began to wonder whether they should be risking their lives for a waning cause," he said.
The growing break over Iraq was dramatized on Thursday in Congress when Representative John Murtha, a former Marine and respected Democratic lawmaker, set off a political firestorm by calling for an immediate withdrawal of US forces from Iraq.
"The American public is way ahead of the members of Congress. The United States and coalition troops have done all they can in Iraq. But it's time for a change in direction," Murtha said.
US military leaders, influenced by the Vietnam experience, have long recognized that the US public support is its "center of gravity," which if tipped could spell disaster in a long war.
Retired general and former secretary of state Colin Powell, author of the Powell Doctrine, favored the use of short, high-intensity wars with clearly defined goals in part because of the difficulty of keeping the public on board in a protracted conflict.
General Richard Myers, the former chairman of the Joint Chiefs of Staff, worried that loss of public support in Iraq could lead to a precipitous withdrawal of forces.
"As a nation, our best weapons are patience and resolve or, in one word, our `will,'" he said before retiring in September.
"We simply cannot afford to lose the will to finish the job at hand," he said.
Since then, public support has continued to slide, and this week the Senate signalled its concern by passing an amendment that called for regular progress reports on Iraq.
"What you see here is that members of both parties are basically running out of patience, and in effect saying they don't care what the consequence of leaving will be," Thompson said.
"When a military officer sees that and understands what it means, it has to have a negative effect on their performance," he said.