WASHINGTON (AP) — The military's top leaders are warning Congress that automatic spending cuts looming in March would force the Pentagon to slash operating budgets, weakening the armed forces and possibly forcing furloughs of 800,000 civilian employees.

The Joint Chiefs say in a letter to Congress that the cuts would create a "hollow force" and would require aircraft to be grounded and warships to return to port.

Troops on the front lines would continue to receive the support they need, despite a cut to the Pentagon's overall budget of more than 7 percent, the chiefs said. But those cuts feel more like 20 percent because they're crammed into the second half of the budget year and funding for uniformed personnel and combat operations is exempt.

"Should this looming readiness crisis be left unaddressed, we will have to ground aircraft, return ships to port, and stop driving combat vehicles in training," said the Jan. 14 letter from Joint Chiefs of Staff Chairman Gen. Martin Dempsey and his colleagues. "We are also now planning for the potential to furlough nearly 800,000 defense civilians who are essential to critical functions like maintenance, intelligence, logistics, contracting, and health care."

The looming cuts are the price to pay for the failure of a 2011 budget "supercommittee" to agree to deficit cuts to replace the automatic reductions.

There's a growing sense of resignation that the country's political leaders will be unable or unwilling to find a way around looming automatic cuts to both the Pentagon and domestic agencies even though there's common agreement that the cuts would threaten the recovering economy.

The biggest impact for the Pentagon would be training and readiness spending.

"The readiness of our armed forces is at a tipping point," the letter said. "We are on the brink of creating a hollow force."

Conservative Republicans, once among the loudest voices against the so-called sequester, now see the painfully large cuts as leverage in their battle to force Democrats into concessions on the budget.

The predictable deadlock — and looming cuts of $85 billion this budget year alone — has the potential to slam the economy, produce sweeping furloughs and layoffs at federal agencies and threaten hundreds of thousands of private sector jobs.

GOP states and congressional districts would bear the brunt of the $43 billion in cuts to defense, however, so it's unclear how much leverage Republicans really would have.