The battlelines are reminiscent of the political struggles during the Korean and Vietnam wars.
Congressional opposition to President Bush's decision to send more troops to Iraq has opened a new front in an increasingly complex war, setting the stage for political battles whose effects may be felt long after U.S. forces have come home.
As if Sunni nationalists, Islamic extremists, foreign fighters and Shiite militias were not enough, Bush must now battle the Democratic leadership in Congress, as well as influential figures in his own Republican party, who oppose his plan to throw 21,500 more troops into the fray.
The battlelines are reminiscent of the political struggles that occurred during two other unpopular wars -- Korea and Vietnam.
Those half-forgotten struggles produced sweeping changes in the U.S. political landscape that persisted long after the wars ended.
Democratic leaders of Congress, fresh from victory at the polls in November, hope to force a vote on the Bush plan in the House and Senate, thereby isolating the president politically.
Although those votes will be nonbinding, Gov. Bill Richardson of New Mexico said on MSNBC he expects them to be followed by measures "that restrict troop funding and all kinds of financial support for the war."
A showdown between Congress and the White House could have far-reaching implications not only for Iraq but for American foreign policy as well.
In the best-case scenario, the prospect of a U.S. military departure might spur Iraq's ethnic and sectarian factions to reach a political settlement to spare the nation further bloodshed.
But it could also encourage Sunni and Shiite extremists to grab as much power as possible before the Americans leave.
Bush's plan is reminiscent of the furor unleashed by President Nixon's 1970 decision to invade Cambodia during the final stage of the Vietnam War.
In seeking to justify his decision, Nixon said the goal was to destroy North Vietnamese sanctuaries in Cambodia and force Hanoi back to negotiations.
Last week, Bush said the reinforcements were necessary to bring stability to Baghdad so that the Iraqis could reach a political agreement.
The 1970 showdown over Cambodia cast a long shadow over American politics.
War Powers Act
The Cambodian invasion enraged Congress, triggering legislative battles that culminated in the War Powers Act of 1973 that sets limits on a president's power to wage war without congressional approval.
Although the act's constitutionality has never been fully tested in court, Congress has invoked it on several occasions.
Vice President Dick Cheney and others have cited the 1973 law as a major element in a series of Vietnam-era legislation that have severely undermined presidential authority to this day.
However, wartime showdowns between presidents and Congress occurred long before the Vietnam conflict.
Those political struggles reflect an ambiguity in the Constitution, which grants Congress the power to declare war but includes no language defining such a declaration.
As a result, many of America's conflicts, including the Civil War, have been fought without a declaration approved by Congress.
In the Civil War, for example, President Lincoln proclaimed Southern states in rebellion and ordered a blockade of their ports, triggering four years of war.
Eight months into the conflict, Congress established a joint committee to oversee the war, holding hearings into Union failures on the battlefield at a time when the very survival of the nation was in peril.
Critics complained at the time that the committee used newspaper leaks to discredit some of Lincoln's generals, undermining Northern morale at a time when Union victory was by no means certain.
Similar complaints were raised again during World War II when a little-known senator from Missouri, Harry Truman, chaired a committee investigating military waste and fraud.
Critics accused Truman of undermining the war effort. But his committee was credited with saving billions of dollars by uncovering waste and war-profiteering.
Truman's efforts won him the Democratic vice presidential nomination in 1944 and propelled him into the White House when President Roosevelt died the following year.
But Truman himself came under criticism for failing to seek a congressional declaration of war when he dispatched troops to South Korea to block an invasion from the communist North.
Public support for the war waned after China entered the conflict, which bogged down in a stalemate. The war became so unpopular that Truman decided against seeking re-election in 1952.
Republican war hero Dwight D. Eisenhower won the White House after promising to halt the fighting, which ended with a cease-fire on July 27, 1953.