By John T. Bennett – 11/18/11 02:51 PM ET

A senior House Republican urged the debt-reduction panel to strike a deal Friday, warning its members to resist the notion that Congress simply can void the deep Pentagon cuts a failure would trigger.

House Armed Services Committee Chairman Buck McKeon (R-Calif.) warned in a letter to the supercommittee co-chairmen that sequestration would bring “immediate, dire and in some cases irrevocable” military cuts.

“Unless we act today, the dismantling of the greatest armed forces in history could begin tomorrow,” he writes.

“Congress can negotiate our way through impasses, but the Department of Defense is required to plan with the budget authority it is given,” McKeon writes in the letter, which was obtained by The Hill.

One leading “misconception is defense can wait a year,” a House Armed Services aide told The Hill on Friday.

By wait a year, the aide was referring to the belief among many on Capitol Hill that Congress can simply use new legislation in 2012 to undo the deep cuts that a supercommittee failure would mandate.

If the special committee fails to find at least $1.2 trillion in deficit cuts, defense spending would be slashed by at least $500 billion over a decade. That would begin with a 25 percent cut in 2013, according to the Pentagon.

The sequestration cuts would not kick in until January 2013.

But because the Pentagon’s 2013 budget request “is due early next year, a failed agreement would force the Defense Department to begin deep and irrevocable cuts to our armed forces next year in order to reap the savings in 2013,” McKeon said. “Once this process is set in motion, it will be difficult — if not impossible — to stop.”

A supercommittee failure would force the “freezing of defense assembly lines and shipyards,” and lead to “hundreds of thousands” of military personnel, industry employees and civilian DOD personnel to be handed “pink slips,” McKeon writes.

The military would not be properly trained and equipped, according to McKeon, who said U.S. forces also would not be able to adequately respond to national security crises or humanitarian disasters.

“The U.S. military’s traditional technical advantage on the battlefield would be eroded,” he writes. The military would shrink, with the ground forces returning to 1940 personnel levels, while it would shed war ships and aircraft, McKeon writes, echoing Defense Secretary Leon Panetta.

“Hope is not a strategy that our senior military leadership can rely [on],” McKeon says, because “they are obligated to plan for the worst-case scenario. They will not wait until December 2012 in hopes that things get better.”

Not everyone sees it that way.

Liberal Democrats and analysts say the annual Pentagon budget nearly doubled since 9/11 and is due for a severe diet.

The Project on Defense Alternatives (PDA) last month called cuts up to $145 billion a year “reasonable.”

“Modest changes to U.S. military strategy and global posture implemented over the next 10 years can reliably offer deficit-reducing savings from the Pentagon budget ranging from $73 billion a year to $118 billion a year,” the PDA stated in an October report. “When these savings are added to other savings available which are not related to strategic change the totals are from $92 billion to $145 billion annually.”

The organization’s report includes a sweeping list of force structure — people and platforms — and strategy changes. PDA recommended reducing the U.S. military footprint in Europe and Asia, while also shrinking the size of America’s strategic nuclear arsenal. It also called for a smaller naval ship and fighter jet fleets.

“None of these strategy and global posture changes would produce a ‘hollow’ force,” PDA analysts wrote, “nor mean that the armed forces of the United States would be anything other than the most-effective, best-equipped and best-trained in the world.”

There is ample room to cut an annual Pentagon budget that has been inflated by 40 percent since 1998 and 30 percent since 2001, PDA said.