WASHINGTON » As the military begins carrying out President Barack Obama’s order to cut force levels in Afghanistan by half over the next year, getting 34,000 troops out is the easy part: Just deliver them to an air field, march them by the hundreds onto transport planes and fly them home.

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.

As Obama delivered his State of the Union message Tuesday night, almost 40,000 armored or other large vehicles remained 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 — they call the effort "retrograde" — 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.

"Afghanistan is not Iraq, and it’s harder," said Lt. Gen. Raymond V. Mason, the Army’s deputy chief of staff for logistics. "No. 1, it’s landlocked. And we have no Kuwait. We have no ‘catcher’s mitt,’ no shock absorber. In Iraq, on the last day, you could still send stuff across the border into Kuwait, and absorb it there."

Mason 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."

Maj. Gen. Kurt J. Stein, commander of the First Theater Sustainment Command, in charge of logistics across the Middle East and Southwest Asia, said there was another land route out of Afghanistan, called the Northern Distribution Network, which runs north through Central Asian republics. But the initial land portion is inconveniently long as it strings toward ports on the Black Sea, the Mediterranean and Atlantic, and presents its own challenges: Railroads are of different gauges, and there are prohibitions on shipping lethal cargo.

Regardless, the movement north from Afghanistan requires passage through the Salang Tunnel, dug into the mountains of Parwan province. The tunnel was a favorite of insurgent ambushes during the Soviet invasion and withdrawal. And, today, U.S. troops are not deployed in Afghanistan’s north.

"About 85 to 90 percent of our equipment is south and east of the tunnel," Stein said, noting that the military never relies on what officers call "a single point of failure" like the lone and vulnerable tunnel on the northern exit route.

Stein, who is based at Fort Bragg, N.C., spoke from his forward headquarters in Kuwait, and also has a second forward hub in Afghanistan. He has traveled the region inspecting ports and has met with transportation executives to accelerate the effort.

In response to queries on the retrograde effort, the military’s Transportation Command said the recent shipments through Pakistani border crossings were a test — "proofs of principle shipments" — to gauge whether the routes can be dependable.

But problems arose. The immense backlog of 7,000 containers that had piled up during the Pakistani closing still had to be reduced. Officials had expected to start dry runs for withdrawal — essentially running trucks through the border crossings — in January. But labor strikes by drivers, squabbles between Afghan and Pakistani custom offices, and internal disputes among Pakistani bureaucracies delayed the initial phase until last weekend. Similar tests of cargo routes are being conducted along the Northern Distribution Network.

Transportation Command officials said major transit hubs, in addition to Karachi, would include ports in the United Arab Emirates, Romania and Spain.

Then there is the question of where the equipment is to go once it leaves Afghanistan. As the withdrawal accelerates, the armed services are negotiating with Pentagon civilians about how best to sort the equipment: 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 because it is damaged or obsolete.

Members of Congress are carefully watching to see how the Pentagon deals with the MRAP troop carriers in Afghanistan, part of a $45 billion urgent effort to build a fleet of armored transports to deflect lethal roadside bombs. The military does not want to pay for hauling them home and fixing them if they are not relevant to future wars. Likewise, officers do not want to scrap them or give them to Afghans or other allies — and then have to buy them again if needed in the future.

Other challenges remain. Even as forces are withdrawn on a timeline set by Obama, sufficient troops must remain to carry out the mission to train and assist Afghan forces — and to facilitate the withdrawal of the gear, and protect that effort.

Senior officials warn that as the number of U.S. troops dwindles, they will be living an increasingly rugged life as dining facilities, gymnasiums and other support services are pulled out in advance of critical combat services that will be the last home.

© 2013 The New York Times Company