WASHINGTON — The amount the U.S. is spending on the Iraq war will surpass the cost of the Vietnam War by the end of the year, making it the second-most-expensive military conflict in American history, behind World War II, according to recent Pentagon figures.

If Congress approves the supplemental funding request submitted this month by the Obama administration, it will add $87 billion to the cost for 2009, apart from amounts for Afghanistan or elsewhere.

Added to the amount spent through 2008, it would mean the Iraq war will have cost taxpayers a total of about $694 billion, according to figures released April 10. By comparison, the Vietnam War cost $686 billion in inflation-adjusted dollars, and World War II cost $4 trillion, according to a Congressional Research Service study.

In Vietnam, U.S. forces at their peak had up to three times as many troops at any one time as in Iraq and suffered 58,000 deaths, more than 12 times as many as have died in Iraq. But experts said there are two broad reasons for the added expense of the Iraq war: people and equipment.

"This is a volunteer military, which is pretty unusual in an extended war," said Stephen Biddle, a military historian at the Council on Foreign Relations. "And people cost more." U.S. officials in Iraq also have relied heavily on private contractors, used to protect diplomats and defend bases, transport provisions and staff essential components such as food services.

A Congressional Budget Office report last year estimated there were 190,000 contract workers employed by U.S. agencies in Iraq — even more than the number of U.S. personnel at the peak of the 2007 troop surge, 160,000 to 170,000 troops. The salaries earned by the contractors were far higher than those of ordinary soldiers.

Medical care in Iraq has been far more expensive, Biddle noted, in part because of a higher survival rate than in past wars.

The cost of the Iraq war has also been driven up by the equipment used. The roadside bombs and sandstorms of Iraq have destroyed very expensive, often high-tech equipment at far more rapid rates than the military expected.