"This is unsustainable," Gen. James Amos, commandant of the Marine Corps, told the lawmakers, referring to curbed spending for training, equipment and force strength that the Marines face. He called those cuts "a formula for more American casualties."
Admiral Jonathan Greenert, chief of naval operations, bemoaned the reduced number of ships the Navy will be able to deploy and said, "We're tapped out."
The automatic cuts, called a sequester, started taking effect this year across defense and many domestic programs. Through 2021, they are slated to cost defense programs a total of $480 billion. They are being imposed because Congress failed to reach a budget compromise for reducing federal deficits.
Because of that, Sen. Mark Udall, D-Colo., apologized to the officers.
"We in Congress created this monster, and we keep dragging you up to the Hill to tell us how much damage it has done," Udall said.
In his testimony, Joint Chiefs Chairman Gen. Raymond Odierno, the Army chief of staff, said the Army is already shrinking its force from a high of 570,000 during the Iraq and Afghanistan wars to 490,000. It would be forced to drop to 420,000 if the sequesters continue, he said.
The Air Force chief of staff, Gen. Mark Welsh III, said he would have to reduce flying hours by up to 15 percent and reduce his service's number of satellites. Greenert said the Navy could have to cut its fleet to as low as 255 ships by 2020, about 30 fewer than today, while Amos said the Marines' 186,000 troops would shrink by the thousands.
Amos also criticized the furloughs of 14,000 civilian Marine Corps employees that were part of the government-wide furloughs forced last month during the budget standoff that resulted in a 16-day partial federal shutdown.
"I'm ashamed about the way they've been treated through the furlough and the uncertainty" it created, he said.
Republicans and Democrats alike, as well as President Barack Obama, have expressed a desire to halt the sequester. But while Republicans want to retain an equal amount of savings by cutting federal spending, Democrats want to do it by also raising some taxes, and the two sides remain at a stalemate.
Some of those differences spilled over into Thursday's hearing.
Sen. Mike Lee, R-Utah, who helped steer Republicans into the fight that produced the government shutdown, quizzed the officers on whether they were conveying their worries about the defense cuts to the White House. He said that while Obama has shown great concern over issues like gun control and his embattled health care program, "I've not heard the same level of concern" about the defense cuts.
Sen. Tim Kaine, D-Va., said such comments were "an effort to kind of avoid looking in the mirror."
Some Republicans, including Sens. John McCain of Arizona and Saxby Chambliss of Georgia, want some of the substitute savings to come from the Pentagon itself, such as by reducing cost overruns for some weapons purchases.