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WASHINGTON - After the 9/11 attacks, the US military found itself ill-prepared for waging war against insurgents in Iraq and Afghanistan, forcing a costly transformation that has left the force exhausted after a decade of combat.
It took only weeks to topple the Taliban in Kabul and Saddam Hussein in Baghdad, but American troops soon found themselves in a protracted battle that they had not trained for against insurgents using crude but lethal weapons.
The Bush administration went to war with an "exaggerated confidence in the efficacy of high-tech warfare to cope with low-tech adversaries and an aversion to the whole concept of nation building," wrote James Dobbins, a former US ambassador now at the RAND Corporation think tank
Having discarded counter-insurgency tactics after the Vietnam conflict, the military's captains and majors had to learn again how to fight militants armed with Kalashnikovs and homemade bombs.
In a bid to avert disaster, the Army and Marines became passionate converts to the guerrilla-fighting doctrine, marking a dramatic shift from conventional training that prevailed in the 1990s after the first Gulf war.
As a result, the wars in Iraq and Afghanistan - which were supposed to be brief affairs marked by a "small footprint" - turned into elaborate nation-building projects, involving tens of thousands of troops and more than a trillion dollars.
The war in Iraq is winding down but the conflict in Afghanistan is approaching its 10th year, with nearly 100,000 US troops on the ground amid plans for a gradual withdrawal over the next four years.
Since 2001, the "war against terror" and operations in Afghanistan and Iraq have cost an astronomical $1.28 trillion, of which 63 percent was devoted to Iraq, according to the Congressional Research Service.
As politicians vowed to prevent another attack after 9/11, defense spending nearly doubled and the intelligence and domestic security bureaucracy expanded, marking the most dramatic change in the "national security state" since the early days of the Cold War.
A new generation of battle-hardened officers rose to the top, led by a cerebral army commander who won acclaim in Washington for salvaging the war effort in Iraq - General David Petraeus.
But years of continuous war have come at a terrible human cost, leaving more than 6,000 US troops dead and 45,000 wounded, along with rising rates of suicide and an epidemic of mental health problems among service members.
Nearly two-thirds of the wars' 1.25 million veterans have returned with debilitating "invisible wounds," including post-traumatic stress and brain injuries, according to a Brown University study.
Senior officers have warned the army is close to the breaking point, and pushed to give soldiers more time at home between tours.
"It's an extremely tired institution. There are limits to how far it can be pushed," said Stephen Biddle of the Council on Foreign Relations.
"So far I think most people have been surprised by how resilient it's proven to be. But no institution is infinitely resilient," he told AFP.
While the all-volunteer military has carried the burden of a continuous state of war, much of the American population has been left untouched by the conflicts, aggravating a gap between soldiers and the civilian world.
The wars also damaged America's image abroad and underscored the limits of US military power, with many Americans now more cautious about intervening abroad -- especially without help from allies.
Before he stepped down as defense chief in June, Robert Gates offered his prediction about the likelihood of another massive ground war using US troops.
"In my opinion," he said in February, "any future defense secretary who advises the president to again send a big American land army into Asia or into the Middle East or Africa should have his head examined.