The Sept. 11 attacks transformed the Pentagon, ravaging the iconic building itself and setting the stage for two long and costly wars that reordered the way the American military fights.
Compared with a decade ago, the military is bigger, more closely connected to the CIA, more practiced at taking on terrorists and more respected by the American public. But its members also are growing weary from war, committing suicide at an alarming rate and training less for conventional warfare.
The partly gutted Pentagon was restored with remarkable speed after the hijacked American Airlines Boeing 757 slammed through its west side, setting the building ablaze and killing 184 people. But recovering from the strain of fighting in Iraq and Afghanistan will take far longer _ possibly decades.
The Pentagon's leaders will have to adjust to a new era of austerity after a decade in which the defense budget doubled, to nearly $700 billion this year.
The Army and Marine Corps in particular _ both still heavily engaged in Afghanistan _ will struggle to retrain, rearm and reinvigorate their badly stretched forces even as budgets begin to shrink. And the troops themselves face an uncertain future; many are scarred by the mental strains of battle, and some face transition to civilian life at a time of economic turmoil and high unemployment. The cost of veterans' care will march higher.
As Robert Gates put it shortly before he stepped down as defense secretary this summer, peace will bring its own problems.
The problem was not peace on 9/11. At the time, the military was focused almost entirely on external threats. Air defenses kept watch for planes and missiles that might strike from afar; there was little attention to the possibility that terrorists might hijack domestic airliners and use them as missiles.
That changed with the creation of U.S. Northern Command in 2002, which now shares responsibility for defending U.S. territory with the Homeland Security Department.
Terrorism was not a new challenge in 2001, but the scale of the 9/11 attacks prompted a shift in the U.S. mindset from defense to offense.
The U.S. invaded Afghanistan on Oct. 7 in an unconventional military campaign that was coordinated with the CIA. That heralded one of the most profound effects of 9/11: a shift in the military's emphasis from fighting conventional army-on-army battles to executing more secretive, intelligence-driven hunts for shadowy terrorists. That shift was important, but it came gradually as the military services clung to their Cold War ways.
Still in debate is how the Taliban, which had shielded Osama bin Laden and other al-Qaida figures prior to the U.S. invasion and was driven from Kabul within weeks, managed to make a comeback in the years after the U.S. shifted its main focus to Iraq in 2003. That setback in Afghanistan, coupled with the longer-than-expected fight in Iraq, showed the limits of post-9/11 U.S. military power.
It also pointed up one of the other key lessons of the past decade of war: It takes more than military muscle to win the peace. It takes the State Department, with its small army of diplomats and development specialists, and other government agencies working in partnership with the Pentagon.
The military grew larger over the past decade, but the growth was uneven. The Army expanded from about 480,000 in 2001 to 572,000 this year, and the Marine Corps grew from 172,000 to 200,000, although both are to begin scaling back shortly. The Air Force and Navy, by contrast, got smaller. The Air Force lost about 20,000 slots since 2001 and the Navy lost about 50,000.
In percentage terms, the biggest growth in the military has been in the secretive, elite units known as special operations forces. They surged to the forefront of the U.S. military's counter-terror campaign almost immediately after the 9/11 attacks, helping rout the Taliban in late 2001 and culminating in May 2011 with the Navy SEAL team's raid on Osama bin Laden's compound in Pakistan. And even though al-Qaida's global reach has been diminished, the increased role of special operations forces is likely to continue.
"It's the most interesting and important change that's likely to endure," Michael O'Hanlon, a defense analyst at the Brookings Institution, said in an interview. "I haven't heard too many people suggest that we can scale back to where we used to be."
The Marines, who had never before fielded forces of this kind, now have 2,600 under U.S. Special Operations Command. The others include the SEALs, the Army Green Berets and Rangers and the Air Force special operators.
In all, those special operations forces grew from 45,600 in 2001 to 61,000 today, according to Special Operations Command.
A decade of war also has produced its military stars. Army Gen. David Petraeus served in command three times in Iraq and once in Afghanistan before accepting President Barack Obama's offer to succeed Leon Panetta as the next CIA director.
Former Iraq commander Army Gen. Raymond Odierno is about to become the Army's top general, and the current Army chief, Gen. Martin Dempsey, who served twice in command in Iraq, is due to replace Navy Adm. Mike Mullen as the next chairman of the Joint Chiefs of Staff.
The military as a whole is viewed more favorably by the American public. A Gallup poll in June found that the military is the most respected national institution, with 78 percent expressing great confidence in it. That is 11 points higher than its historical Gallup average dating to the early 1970s.
The new technological star is the drone aircraft, like the Predators that surveil the battlefield and fire missiles at discrete targets. Their popularity has spawned an effort to field unmanned aircraft to perform other missions, such as a long-range bomber and even heavy-lift helicopters.
Robert Burns can be reached on Twitter at http://twitter.com/robertburnsAP