FORWARD OPERATING BASE SALERNO, Afghanistan — It’s a process that plays out every year at U.S. bases across Iraq and Afghanistan: A new unit arrives, an old unit leaves, and for a brief and important time, they overlap.

But with the U.S. set to roughly double its forces in Afghanistan this year, the incoming troops will vastly outnumber those on their way out, posing a massive logistical challenge in a theater where American troops have been spread thin.

And as the new units seek to turn the tide against a growing insurgency, making the most of their overlapping time with outgoing troops will be an early key to success.

The changeover, known in military parlance as a "RIP/TOA," for "relief in place and transfer of authority," is already beginning to play out in parts of eastern Afghanistan.

The current U.S. infantry brigade nearing the end of its yearlong deployment in a vast stretch of territory southeast of Kabul will hand off half its area to a brigade of the 10th Mountain Division this month. Later this spring, it will turn over the remainder to a brigade from the 25th Infantry Regiment.

"It’s challenging enough to do one RIP, but we’ll have less than a month between the two," said Maj. John Bowman, the executive officer of the outgoing 4th Brigade, 101st Airborne Division, known as Task Force Currahee.

If any unit is prepared for the process, though, it’s this one. Commanders have been forced to frequently shift units, leaving Currahee in a state of constant motion since it arrived last spring.

One of its infantry battalions was initially assigned as a country-wide quick reaction force, then moved to take control of Paktia province along the border. Another battalion spent its first nine months in Gazni province before moving to Paktika.

A Polish Battle Group assigned to the brigade switched out with another Polish unit, then moved provinces. When the 10th Mountain unit arrives, the understrength battalion currently just south of Kabul will move to the border. "We’ve been in a constant state of RIP/TOA," the brigade’s spokesman, Maj. Patrick Seiber, said.

But the brigade-level handoffs coming in the next few months are far more involved. Currahee’s logistics officer, Maj. Dan Heape, began in July the process of lining up the Air Force assets and local trucking needed to move the brigade’s 3,500 troops and their equipment.

Meanwhile, ground units have been busy building up infrastructure at bases that will see an increasing number of troops.

And the process involves much more than logistics. Officers in the outgoing unit, from the brigade commander and his staff officers down to company commanders and platoon leaders, spend months preparing detailed reports on their areas — outlining everything from local leadership and suspected insurgents to significant firefights and successful tactics.

The process culminates in a typically two-week period of joint operations, with the outgoing unit in the lead the first week and the new unit taking over during the second week.

Still, the transition remains a difficult time. Units usually suffer their highest rate of casualties in their first months on the ground and face a steep learning curve, no matter how well they were prepped by their predecessors.

"It’s a very complex operating environment," Bowman said. "It really takes two or three months to learn your area of operations."

But officers say the new units will benefit from several factors.

The division headquarters that oversees U.S. operations across the east will remain until summer, fostering a sense of continuity and giving the incoming troops "some insight into why things are the way they are," said Capt. Jim Raines, Currahee’s plans officer.

The 10th Mountain unit will arrive in the middle of winter, which as in years past has produced a lull in fighting — though the weather presents its own challenges.

"Weather is really our greatest enemy in this process right now," Heape said. "But we’d rather fight the weather than the enemy."

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