After the 9/11 attacks, the US mili-tary 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 pro-jects, 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 per cent were devoted to Iraq, according to the Congressional Research Ser-vice.

As politicians vowed to prevent another attack after 9/11, defence 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 “invis-ible wounds,” including post-traumatic stress and brain injuries, according to a Brown University study.

Senior officers have warned that 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 remarked.

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 defence 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 Defence Secret-ary 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.”

Defining features of the post-9/11 wars

Predators: An unmanned, propeller plane equipped with cameras, precision-guided bombs and missiles, the Predator has allowed the US military to spy from the sky for hours on end and attack terror suspects without putting troops at risk. The Predators, part of a new wave of robots that are changing the face of war, became the workhorse of the campaigns in Iraq and Afghanistan as well as the CIA’s covert drone campaign in Pakistan against Al-Qaeda and the Taliban.

The US military has 269 of the aircraft but is replacing them with an improved model, the Reaper, which can fly twice as fast and cruise at higher altitudes.

The Predators and Reapers are among thousands of unmanned aircraft and ground robots now deployed by the American military, including the giant Global Hawk, a spy jet with a 116-foot wingspan that can fly at high altitude, replacing the U2 plane born in the Cold War era.

Mraps: Homemade bombs, the number-one killer of troops in Iraq and Afghanistan, have proved lethal against the military’s standard vehicle, the Humvee, which has a vulnerable, flat bottom. Under former Defence Secretary Robert Gates, the Pentagon started buying large numbers of tougher trucks, designed to withstand blasts from roadside bombs, known as Mine Resistant Ambush Protected vehicles. The MRAPs’ V-shaped hulls help deflect the force of explosions and the vehicle’s higher chassis keeps troops further from the main force of the blast from improvised explosive devices.

Commanders believe the MRAPs have helped save thousands of soldiers’ lives, and figures show the number of casualties from IEDs dropped more than 80 per cent after the introduction of the vehicles. The US military now has 27,740 of the vehicles, including more than 8,000 modified MRAPs for Afghanistan’s tougher terrain and bad roads.

High-tech gear: The wars in Iraq and Afghanistan gave US soldiers a new look, with night vision goggles attached to helmets, sophisticated gun sights, knee-pads and digital camouflage.

The simple flak jackets worn by troops at the beginning of the Iraq war in 2003 were fortified with ceramic plates and eventually replaced with more elaborate personal armour, with collars protecting the neck and side panels.

Special operators: The “war against terror” elev-ated the role of elite special operations forces, the heavily-armed, highly-trained troops that move in small teams. Often sporting beards to keep a low profile, the commandos became a crucial weapon for manhunts and intelligence ­gathering. In Afghanistan in the weeks after the 9/11 attacks, special forces units and covert CIA operatives – some of them on horseback – deployed alongside Northern Alliance forces to push the Taliban from power.

In Iraq and Afghanistan, ­commanders relied on the special forces units to carry out numerous night raids to capture or kill militant leaders in a bid to break the back of the insurgencies.

Members of the Navy Seals’ famed Team Six became national heroes when they led a daring night-time raid deep into Pakistan, gunning down Al-Qaeda chief Osama bin Laden on May 2, 2011.