KANDAHAR AIRFIELD, Afghanistan - Facing a tight withdrawal deadline and tough terrain, the U.S. military has destroyed more than 170 million pounds of vehicles and other military equipment as it rushes to wind down its role in the Afghanistan war by the end of 2014.

The massive disposal effort, which U.S. military officials call unprecedented, has unfolded largely out of sight amid a debate inside the Pentagon about what to do with the heaps of equipment that won't be returning home. Military planners have determined that they will not ship back more than $7 billion worth of equipment - about 20 percent of what the U.S. military has in Afghanistan - because it is no longer needed or would be too costly to send home.

That has left the Pentagon in a quandary about what to do with the items. Bequeathing a large share to the Afghan government would be challenging because of complicated rules governing equipment donations, and there is concern that Afghanistan's fledgling forces would be unable to maintain it. Some gear may be sold or donated to allied nations, but few are likely to be able to retrieve it from the war zone.

Therefore, much of it will continue to be shredded, cut and crushed to be sold for pennies per pound on the Afghan scrap market - a process that reflects a presumptive end to an era of protracted ground wars.

"We're making history doing what we're doing here," said Maj. Gen. Kurt Stein, head of the 1st Sustainment Command, who is overseeing the drawdown in Afghanistan. "This is the largest retrograde mission in history."

The most contentious part of the effort involves the disposal of Mine-Resistant Ambush-Protected vehicles, the hulking beige personnel carriers.

The Pentagon has determined that it will no longer have use for about 12,300 of its 25,500 MRAPs scattered at bases worldwide, officials said. In Afghanistan, the military has labeled about 2,000 of its roughly 11,000 MRAPs "excess." The majority of the unwanted vehicles - which cost about $1 million each - will probably be shredded, officials said.