FORWARD OPERATING BASE SALERNO, Afghanistan — U.S. reinforcements have begun to filter into eastern Afghanistan ahead of what is widely viewed by the military as a critical season in the seven-year war against a resurgent Taliban.

Though many of the 20,000 to 30,000 additional U.S. troops expected to arrive this year will head for Taliban strongholds in the south of the country, thousands will be used to bolster efforts around American-patrolled hotspots in the east — especially along the border with Pakistan — as well as dramatically strengthen the U.S. presence in several interior provinces where insurgent activity has increased.

U.S. troops currently make up about half the 60,000 international forces in Afghanistan, with American efforts focused on the east. The additional troops expected to arrive this year will roughly double the American contingent.

The arrival of the additional troops comes after two years of dramatically escalating violence, with 2008 by far the deadliest year of the war for U.S. and international troops. A recent independent assessment of the war concluded that the Taliban now has a permanent presence in three-quarters of the country, up from just over half in 2007.

While American commanders are quick to point to the differences between Afghanistan and Iraq, the incoming units will undoubtedly face heightened expectations for the war, especially after dramatic security gains followed the U.S. "surge" of about 30,000 troops into Iraq in early 2007.

The first major group of reinforcements — about 3,500 troops from the 3rd Brigade, 10th Mountain Division — began arriving this month. Most of the brigade will take positions in Wardak and Logar provinces southwest of Kabul, roughly tripling U.S. force levels there compared to 2008 numbers.

With U.S. troops focused on the border in an effort to cut off insurgents from their bases in Pakistan, violence has risen in several provinces surrounding Kabul, where U.S. commanders have in some cases been able to field little more than a token force. In Logar, for example, a single platoon of fewer than 50 U.S. troops patrolled an area of roughly 700 square miles this summer, and officers described an active and growing Taliban presence.

The arrival of the 10th Mountain brigade will allow the troops stationed in Logar and Wardak — three companies and a battalion from the 4th Brigade, 101st Airborne Division — to reinforce the brigade’s main effort along the border. The brigade, known as Task Force Currahee, will be replaced by the 4th Brigade, 25th Infantry Division later this spring.

"The effects they should be able to achieve — we’ve had great success — but I think they should really be able to accelerate what we’ve started," said Task Force Currahee’s executive officer, Maj. John Bowman. "The battle space will almost be cut in half."

A single battalion of 10th Mountain troops will be sent to northeast Afghanistan to reinforce the 3rd Brigade, 1st Infantry Division, which lost one of its battalions to the fight in the south when it arrived last summer.

Fighting in much of the east has slowed since November, as snow and cold weather have settled in. U.S. commanders have promised a "winter offensive" in an effort to prevent insurgents from preparing for renewed fighting, and some have attributed the decline in violence to the effects of American operations. Meanwhile, a wave of bombings has continued in southern Afghanistan, with 37 coalition troops killed since the beginning of December.

In addition to the U.S. reinforcements, several other plans to increase security forces are in the works, including a plan to nearly double the size of the Afghan Army. Commanders hope a broad effort to train and equip the Afghan Border Patrol will also pay dividends by the summer.

U.S. and NATO commanders have long pressured European allies to step up their efforts in Afghanistan, but with few firm commitments so far.

And while President-elect Barack Obama has consistently signaled a strong commitment to the war, it’s not clear how many more U.S. troops, if any, might be sent after this year’s build-up. Secretary of Defense Robert Gates, who has agreed to remain Pentagon chief under Obama, last month said military force alone could not win the war, noting that the Soviet Union committed 120,000 troops to Afghanistan in the 1980s and still lost.

Afghanistan remains one of the poorest countries in the world. Seven years after the U.S.-led invasion, broad swaths of the country still lack electricity, running water and access to basic health care and education. Aid groups have warned that high food prices and a poor growing season have left some 8 million Afghans — or 30 percent of the population — on the brink of starvation this winter.

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