WASHINGTON -- As the military begins carrying out President Obama's order to cut force levels in Afghanistan by half over the next year, getting 34,000 troops out is the easy part.

But after 11-plus years of war, the accumulated U.S. hardware in Afghanistan amounts to more than 600,000 pieces of equipment valued at $28 billion. In that arsenal are systems that always present challenges to international shipping, including MRAP mine-resistant troop transports and Stryker infantry fighting vehicles, each built with tons of armor, and heavy tractor-trailers and tankers.

So far, the heavy vehicles have all been shipped out by air because Afghanistan is landlocked, it has a primitive road system and the Taliban remain strong in many parts of the country. But the real problem to withdrawing from Afghanistan is the same one that has helped make fighting there so difficult: the tenuous relationship with neighboring Pakistan, which offers the cheapest land route to the closest seaport but through border crossings that are unreliable.

Logistics officers are only too mindful that Pakistan closed the routes after U.S. airstrikes killed 24 Pakistani soldiers at an outpost on the Afghan border in November 2011. The routes were reopened only last July after Washington apologized. But U.S. officials hope that up to 60 percent of the hardware in Afghanistan can be sent out by way of Pakistan.

Almost 40,000 armored or other large vehicles remain in Afghanistan. The military has a goal of bringing out 1,500 of them every 30 days, a target it can reach -- in a good month -- by air. But there are just 22 months until the U.S.-led combat mission ends in December 2014. It is going to be a challenge, requiring Pakistan to open those border crossings permanently, and it is going to be expensive.

The military, of course, is practiced at massive movements of material, and most of the senior officers involved in pulling equipment from Afghanistan did the job in Iraq. But these officers stress that Iraq offered a sophisticated roadway system and flat terrain. Even more helpful, Iraq borders Kuwait, where U.S. equipment could be stored in large numbers at American bases, and then shipped home on a relatively unhurried timeline.

Lt. Gen. Raymond V. Mason, the Army's deputy chief of staff for logistics, said re-establishing with certainty a pair of ground crossings into Pakistan would allow a larger volume of equipment to make a faster exit from Afghanistan; the gear would be driven to Karachi, and then shipped by sea back to the United States. He said the first containers of hardware leaving Afghanistan had been driven into Pakistan just in the past few days. "That's the good news," he said. "But it is still very fragile."

Then there is the question of where the equipment is to go once it leaves Afghanistan. Some weapons and hardware must be brought home, repaired and redistributed across the fighting force; some will be sold or donated to the Afghans or other partner nations in the region; and some will be scrapped.