WASHINGTON – Ten years ago, the United States massed a traditional military force behind sand-berm walls separating Kuwait from Iraq.

Hundreds of thousands of soldiers, sailors and Marines in camouflaged chemical warfare uniforms sat in or stood near idling trucks, personnel carriers and Humvees. They were waiting for an hours-long barrage of artillery and bunker-busting bombs to soften up whatever forces Saddam Hussein had still willing to defend the border, before rolling forward to Baghdad.

But on the moment – March 20 along that border, March 19 in the United States – that the American military was preparing regime change in Iraq, the evolution within the military in the decade since has also been significant.

”We started with the march-up to Baghdad, and that was very much a traditional fight,” said retired Col. Michael Starry, who now concentrates on how the Army must adapt in the future.

Starry is with the Army Capabilities Integration Center, an in-house think tank that studies how the service adapts to the evolving landscape of modern warfare.

Bill Rittenhouse, who works in a similar position with the Army think tank, added: ”Then the whole face of it began to change. There clearly was a point after which the Iraqi military ceased to be a problem. That’s when we recognized the development of an insurgency.”

Some of the changes have been more obvious.

In 2003, the Department of Defense budget totaled $437 billion. By 2009, it had climbed to $691 billion. This year’s budget will be difficult to predict because of the lack of an approved budget and the effects of automatic cuts. But it should be around $580 billion.

Beyond the dollars being spent, though, has been a shift in philosophy. American troops arrived in Iraq as warriors. By 2011 when the last forces pulled out, they left as nation-builders, negotiators, cops, civil workers and social workers, as well.

The rules of engagement were aggressive in the early days.

In March 2003, a Marine captain stood in front his company, showing images of the wide range of uniforms and garb the enemy might wear. After each image, he would ask for the correct response, and the answers were so consistent that halfway through the lecture, Marines began shouting in chorus, ”One in the heart, one in the head,” as each new image appeared.

The message was clear: Identifying the enemy won’t be easy, so be prepared to kill a wide range of people.

Less than two years later, a gunner on an Army helicopter kept a trash bag full of Beanie Babies next to him as he raced above the farms and villages of eastern Iraq. Occasionally, he reached into the bag and tried to drop toys into groups that might contain children.

The message was clear: Identifying and making friends won’t be easy, so be prepared to work hard at it.

Brig. Gen. Michael Lundy, deputy commanding general of the Combined Arms Center-Training at Fort Leavenworth, Kan., said that a military that had prepared for decades to face off against the old Soviet Union was now looking at engaging small groups that would strike, then disappear. Victory was as much about building support among the local population as superiority in arms.

Training had to reflect such needs, he said.

”We realized we were not going to be able to see everything in the higher echelons quickly enough,” Lundy said. ”We decided we would need our junior officers to be able to react, but to stay inside the overall plan.”

So information that once was reserved for the highest levels now needed to be shared among lower ranks, said Henry Franke, a military analyst with the Army. To prepare for insurgent tactics – from the use of mortars rounds, to car bombs, to roadside bombs – lower-ranking officers in the field had to develop a strategy of how to react.

”Iraq, more than Afghanistan, forced us to develop a structure where a smaller unit could own territory,” Starry said.

In Iraq, there was no one-size-fits-all approach to the fight. Each service member also needed to be trained to take on more responsibility than in previous wars. The squad became the decisive force on the battlefield. Yet at the same time, each branch of the military – the Army, the Navy, the Air Force and the Marines, historically used to pursuing their own agendas – had to work together.

A decade later, as the rules of warfare change and new enemies emerge, Lundy said that the military needs to keep those lessons in mind.