The confrontation between Congress and the White House over Iraq is developing into perhaps the most heated confrontation since Vietnam over one of the most basic aspects of the US Constitution – its allocation of the power to make war.
In history, Congress often has fallen short of its goals when it attempts to rein in or change the executive branch's conduct of war. Presidents have many ways of forging ahead despite political and legislative resistance.
But in some instances, lawmakers have played a pivotal role in ending US involvement. Their power to raise questions, via hearings and investigations, can be almost as important as their ability to cut off funds.
In part, that's what happened with Vietnam, though Congress today is not as roiled as it was in the late 1960s and early '70s. "We're not there yet," says Julian Zelizer, a Boston University historian and expert on Congress and Southeast Asia.
If nothing else, the words now being tossed back and forth over Iraq in Washington are becoming increasingly impassioned. On Monday, Vice President Dick Cheney in a speech to the American Israel Public Affairs Committee lambasted members of Congress who back legislative attempts to place restrictions on what President Bush can do in Iraq.
"When members of Congress pursue an antiwar strategy that's been called 'slow-bleed,' they're not supporting the troops. They are undermining them," said Vice President Cheney.
In reply, Senate majority leader Harry Reid (D) of Nevada said in a statement that America is less safe today because of the war. Mr. Bush "must change course, and it's time for the Senate to demand that he do it," the senator said.
Yet on Tuesday congressional Democratic leaders removed from a military spending bill for the war in Iraq a provision requiring that Bush gain approval from Congress before making any move against Iran. Conservative Democrats, as well as some other lawmakers, had objected that this provision might lessen US negotiating leverage, as well as possibly embolden Iran to be more regionally aggressive.
The provision's removal shows how difficult it may be for House Speaker Nancy Pelosi (D) of California and Senate majority leader Reid to keep Democrats together in the coming debates over further Iraq restrictions. It may also show how hard it is for Congress to strike a consensus over difficult questions of war and peace.
The bare outlines of the constitutional allocation of power in regard to war are well known to anyone who has passed a US civics course. Congress has the authority to declare war, and to raise and support military forces. The president is the commander in chief.
But the importance of the stakes, as well as the way those powers can clash, virtually ensures continual struggle between the executive and legislative branches of government.
New England and the Federalist Party, for instance, bitterly opposed the War of 1812. They managed to force the resignation of President James Madison's secretary of War, but in the nationalist surge that followed the war's end, the Federalist Party collapsed.
During the Civil War, Congress held extensive hearings to try to push Union generals to further action. President Abraham Lincoln had a difficult time simply keeping Republicans in Congress united.
Prior to the Japanese attack on Pearl Harbor, antiwar sentiment in Congress limited President Franklin Roosevelt's ability to intervene on the side of the British.
Congress played a significant role in the Vietnam conflict. While it did not directly end US involvement, between 1964 and 1975 many legislators forced discussion of difficult questions about the war, publicly challenging the administration, notes Julian Zelizer in an article on the subject in The American Prospect.
In 1969, Congress used the power of the purse, passing a bill that prevented use of US money to fight in Laos or Thailand. That provision was eventually stretched to cover Cambodia. In 1974, after US combat troops were withdrawn, Congress cut aid to the South Vietnamese government almost in half, to $700 million. Saigon fell in the spring of 1975.
Fighting with Congress over Iraq is already taking a political toll on the administration, says Mr. Zelizer. While the White House may be able to ward off congressional attempts to limit what troops can do, or how many can be deployed, its room to maneuver may be getting smaller.
"Even though right now it looks like they are getting what they want, it is not the presidency we saw in 2004 and 2005," says the Boston University historian.
Most legal scholars agree that Congress has the power to cut off funds for the war, or for a certain war activity, if it can muster the votes.
Yet Congress may not be able to try to change how a war is fought, or micromanage the president's decisions.
"Decisions involving the conduct of war, including where to move troops, whether to reinforce troops, whether to move troops from one hill to another, are vested exclusively in the president," said Robert Turner, cofounder of the Center for National Security Law at the University of Virginia, in a January appearance before the Senate Judiciary Committee.
• Wire service material was used in this report.