Defense Secretary Robert Gates surprised lawmakers Thursday by announcing that the Pentagon would cut spending by $78 billion over the next five years.

The cuts will force the Army and Marine Corps to reduce the number of troops on active duty and eventually freeze military spending for the first time since the terrorist attacks of Sept. 11, 2001.

“The surprise announcement from Gates was a reminder for the military establishment – which has benefited from a gusher of new money over the past decade – that it will not remain exempt from painful austerity measures that federal lawmakers say will be necessary to control the soaring national debt,” The Washington Post‘s Craig Whitlock noted.

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The $78 billion comes in addition to $100 billion in cuts that Gates directed the Pentagon to find in May. In a Thursday news conference, the defense secretary said that the armed forces had successfully found $100 billion in savings over the next five years and the Army, Navy and Air Force would get to reallocate that money to combat-related projects.

As a part of the cuts, the Pentagon is expected to end the Marine’s Expeditionary Fighting Vehicle, a 40-ton amphibious landing craft being developed by General Dynamics Corp.

A ground-launched missile system being developed by Raytheon Co. is also on the chopping block.

The number of Marines will be cut by 10,000 to 15,000 and the Army will reduce its forces by 27,000, Gates said.

Republicans had an immediate negative reaction to the announcement.

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“We are fighting two wars, you have China, you have Iran: Is this the time to be making these types of cuts?” House Armed Services Committee Chairman Buck McKeon (R-CA) told The Hill.

The savings nearly equal the $100 billion in spending cuts promised by Republicans this year. However, GOP sources told The Huffington Post’s Howard Fineman that the real goal is to cut about $30 billion.

CLARIFICATION: (Edited to provide further details.) The $78 billion in defense cuts are separate from $100 billion in “efficiencies” identified by the Pentagon. The $100 billion will be reinvested into other Pentagon programs and will not result in spending cuts; the $78 billion will be cut from the Pentagon’s budget over a five-year period beginning in the 2012 fiscal year. Because the Pentagon originally projected budget increases for these years, the cuts won’t mean that the Pentagon’s budget will shrink in total dollar terms — only that the rate of growth will slow. The fiscal year beginning this year will see a base Pentagon budget of $553 billion, up from $549 billion last year.