The Pentagon's top civilian and military leaders implored Congress Wednesday not to cut too deeply into military spending, plunging into a heated political debate over how to curtail defense costs without imperiling U.S. strategic interests at a time the military is fighting two wars.

"We shrink from our global security responsibilities at our peril," Defense Secretary Robert Gates told the House Armed Services Committee. "Retrenchment brought about by short-sighted cuts could well lead to costlier and more tragic consequences later -- indeed as they always have in the past."

Gates and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, are trying to strike a balance between constraining future defense budgets as part of a government-wide effort to reduce budget deficits and preserving U.S. military power. One of the more controversial elements of the administration' proposed 2012 budget is a plan to reduce the size of the Army and Marine Corps starting in 2015.

Gates said the Pentagon is asking for $553 billion for the budget year that begins Oct. 1, plus $118 billion in war costs. He also is asserting that the Pentagon will face a crisis if the Congress does not pass a new defense budget for the current year or passes one with significantly reduced funding. So far this year the Pentagon has been required to operate on last year's budget. He said the Pentagon can get by with as little as $540 billion this year.

The committee chairman, Rep. Howard "Buck" McKeon, R-Calif., told Gates and Mullen that it is premature to be reducing the size of the Army and Marine Corps.

"I cannot in good conscience ask them to do more with less," McKeon said.

Marshaling his counter-argument, Gates told the panel that the Army and Marine Corps can be reduced "with minimal risk," given that far fewer troops will be deployed abroad when forces are drawn down from war zones. He said that one year ago 190,000 troops were stationed in Iraq or Afghanistan, and that by the end of this year the total will be slightly below 100,000. That will mean more time at home between overseas deployments for much of the force, Gates added.

Noting also that the reductions would not start until 2015, Gates said the date is far enough in the future to enable the Pentagon to adjust if circumstances change.

Gates said the major assumption upon which the troop cut decision was based is that the U.S. will have ended its combat role in Afghanistan by the end of 2014. The Pentagon probably will know by late 2012 or early 2013 whether that assumption is valid, and if it is not, the troop cut decision could be reversed, he said.

Gates added that the Marine Corps' top leadership recommended that their service be shrunk. They believe that once they are out of Afghanistan, the Corps will be too large to fulfill its traditional missions, he said.

In his testimony, Mullen said he supported the administration's controversial proposal to increase some health care costs for working-age military retirees. He called the increases "modest and manageable," and noted that the heads of all the armed services support the proposed increases.

Mullen also said it would be a mistake to reduce U.S. military aid to Egypt in the aftermath of its revolution.