'It tears you up every time,' said Army Col. Paul Bricker, recounting the death of one of his most experienced pilots, whose Apache helicopter crashed in the southern Afghan desert May 22, just days after the start of his tour. Bricker's 82nd Combat Aviation Brigade, part of the storied 82nd Airborne Division based in Fort Bragg, N.C., is the leading edge of the largest U.S. troop buildup since the start of the Afghan war in 2001. At Kandahar airfield, its new headquarters, the unceasing roar of combat aircraft and the dust-laden din of new construction are testament to a push by the Obama administration to alter the course of a troubled conflict. Most of the 17,000 combat troops ordered here by President Barack Obama will be in place by mid-July -- nearly all of them fanning out across southern Afghanistan, the heartland of the Taliban insurgency. The south also is home to punishing terrain and climate conditions, a desert landscape where sandstorms howl and summertime temperatures soar to 120 degrees. An additional 4,000 American troops, who will be responsible for training Afghan soldiers and police officers, are due to arrive by August. That will push U.S. troop strength above 60,000, a reflection of the shifting emphasis from Iraq to Afghanistan, where Western strategists acknowledge that more than seven years of fighting has yielded essentially a stalemate. American military officials know full well that the summer could exact a heavy toll in lives, as the arriving forces make it a priority to push into areas where the Taliban has had free rein. But U.S. commanders also view the coming few months as a potential 'game-changer,' as Bricker put it -- an opportunity not only to wrest territory from the insurgents but to keep it secure for long enough that Afghanistan's fragile brand of governance can take hold. That task is seen as particularly important before Afghan elections Aug. 20, which are seen by Western observers as a milestone on the country's bumpy road to some semblance of democracy. American troops, previously concentrated in the east of Afghanistan, are taking up new positions in some of the most volatile areas of the south and west. A U.S. Marine expeditionary brigade from Camp Lejeune, N.C., is headquartered in Helmand province, which has been the scene of some of the war's fiercest fighting. Several thousand of those Marines have arrived, taking over formal command last week, and their numbers are expected to swell to 8,000 in coming weeks. (Begin optional trim) Thinly spread British troops had been bearing the brunt of combat in Helmand, which is Afghanistan's biggest opium-producing province. Drug money fuels the insurgency, buying weapons and luring fighters into the ranks. Helmand is also a prime infiltration route for Taliban foot soldiers who move freely from bases in southern Pakistan. The Marines, though, hope their numbers will be sufficient to make a difference in districts such as Nad Ali, a Taliban command-and-control hub in Helmand's northwest where Afghan and Western troops have long fought to make headway. (End optional trim) 'We know this will be a challenging assignment,' said Capt. Bill Pelletier, the Marine brigade's spokesman. 'The Taliban are an adaptive enemy force.' Although Western military officials are careful to avoid characterizing the buildup as a 'surge,' the hubbub of activity in the south has not been lost on the Taliban leadership. But insurgents profess unconcern. 'They must use the roads, and on the roads we will kill them,' said a local Taliban commander in Helmand, speaking by phone from an undisclosed location. He was referring to the insurgents' weapon of choice: IEDs, or improvised explosive devices. Roadside bombs are responsible for about 70 percent of the Western troop casualties in the south, commanders say. Three U.S. soldiers were killed Thursday when their vehicle was attacked by a roadside bomb followed by a small-arms attack, according to a military statement. On Monday, two roadside bombings in eastern Afghanistan killed four American soldiers. The Americans hope that the arriving contingents' air power, which represents a quadrupling of aircraft based at Kandahar, will prove a pivotal weapon in the fight against roadside bombs, among other things. 'We've got lots more eyes on them now, and they'll get to understand that very soon,' said Bricker, the aviation brigade commander. A bigger helicopter fleet allows closer surveillance of the south's long stretches of desert roadways, where such signs as disturbed earth or nighttime activity can pinpoint the location of bombs. Still, the presence of such large numbers of troops and aircraft could result in higher casualties, not only for U.S. forces but for Afghan civilians as well. Coalition forces rely on air power, but it can be difficult at times to distinguish civilians from combatants. Afghans were infuriated last month by what might have been the war's worst instance of civilian deaths caused by Western forces: as many as 140 villagers killed in U.S. air strikes in Farah province, according to the Afghan government. The American military puts the figure at 20 to 30. As is always the case with such a sizable deployment, some deaths will occur from causes other than direct clashes with insurgents. The military is investigating the cause of the May 22 Apache crash in Oruzgan province that killed the highly decorated pilot in Bricker's unit, Chief Warrant Officer Brent S. Cole. The other crew member was seriously injured but survived. In Kandahar, the south's main city, there is hope and skepticism on the part of Afghans in the face of this new U.S. push. Particularly in outlying areas, residents say, foreign forces often temporarily establish a zone of safety but insurgents return when the troops are occupied elsewhere. 'Of course we would like our part of the country to be peaceful, after so much time,' said Najib Ali Khan, a wheat merchant in the city. 'And, of course, the Taliban will sometimes decide to leave a particular area. But they are never gone for good.' 