A quarter of a trillion dollar gap exists between what the Pentagon needs to do over the next five years and the money it's going to get under sequestration, Sen. John McCain said.

"What this means is that, over the next five years, our nation must come up with $250 billion just to pay for our current defense strategy and our current programs of record," McCain, R-Ariz., said as he opened a Senate Armed Services Committee hearing Thursday to take testimony from the national's top military officers.

"$250 billion just to do what we are planning to do right now, which I think many of us would agree is insufficient to meet our present — let alone our future — challenges."

In opening statements, all four service chiefs said the spending constraints imposed by the Budget Control Act of 2011 were forcing decisions that robbed future readiness, to maintain the combat capability of the current force.

And when polled by McCain, all the chiefs, to a man, agreed that unless sequestration is repealed, the U.S. military will not be able to meet future threats and defend the nation.

Army Chief of Staff Gen. Mark Milley testified the Army has been forced to shift money from important modernization initiatives in order to keep front line troops fully combat ready.

"To mitigate the risk of deploying an unready force into future combat operations, the Army [is] fully funding and prioritizing readiness over end-strength modernization," Milley said. "In other words, we are mortgaging future readiness for current readiness.

"The only thing more expensive than deterrence is actually fighting a war, and the the only thing more expensive than fighting a war, is fighting one and losing one. This stuff is expensive. We're expensive. We recognize that, but the bottom line is it's an investment that is worth every nickel."

Adm. John Richardson, the chief of naval operations, said the Navy is also making budget tradeoffs.

"Funding levels require us to prioritize achieving full readiness only for our deploying units. These are ready for full-spectrum operations, but we are compromising the readiness of those ship and aircraft that we will have to surge to achieve victory in a large conflict," Richardson said.

The deadlocked Congress has been unable to muster the political support to repeal the spending limits, which have five years to go.

GOP presidential candidate Donald Trump had promised to end what he called "the defense sequester" as soon as he's in office, something that could be possible if Republicans retain control of the House, and increase their majority in the Senate.

McCain noted that with congressional action, the spending constraints will remain in effect through the entire term of the next president.

"In short, we lied to the American people," McCain said. "The Budget Control Act and sequestration have done nothing to fix our national debt. ... And what's worse, the people we have punished for our failure are none other than the men and women of our armed services, and many other important agencies."