This is the last of a three-part look at the evolution of the American military.

Struggling to wind down the longest war in American history, retiring Army Chief of Staff Gen. George W. Casey Jr. lobbed a bomb. He described a future rife with “persistent conflict.” The phrase, which was long studied and carefully forged over several years by the Pentagon, has become a subject of speculation in both military and civilian circles, as America’s commitment to Iraq winds down, Afghanistan promises to draw down troops in July, and a new combat mission in Libya is under way, roiling the already tumultuous Middle East.

(Having fulfilled his three-year commitment, Casey stepped down on April 10, 2011, and was replaced by General Martin E. Dempsey, the commanding general at the U.S. Army Training and Doctrine Command.)

The term represents a link in a chain of events that began in 1973 with the creation of an all-volunteer military. Less than 20 years later, those volunteers had evolved into a well-oiled professional fighting machine, able to project power anywhere around the globe.

The bloodbath in the Balkans gave rise to a second link, when the U.S. contributed troops and aircraft to separate the warring factions in Bosnia and Herzegovina. Large numbers of private contractors were hired to support the military. Those numbers multiplied in Iraq and Afghanistan, as more and more of the military’s capabilities were privatized. Without professionals and privatization, “persistent conflict” would be unlikely.

In hindsight, the nation may have been preparing for a future of “persistent conflict” since Bosnia in 1995. A one-year commitment in the Balkans ended nine years later in 2004, overlapping two other wars.

Casey began talking publicly about “persistent conflict” in an August 2007 speech before the National Press Club in Washington. His new job had begun that April as a presidential campaign ramped up and the situation in Iraq wound down.

During the first month at his desk, Casey saw four bombings in Baghdad kill 198 people. That same April, a suicide bomber entered the Iraqi Parliament building, killing one representative and wounding 22. The attack had taken place “inside the wire,” in the fortress-like Green Zone that held the American embassy, the Iraqi government and all of the major news bureaus.

Before the end of his first month on the job, Casey’s boss, Secretary of Defense Robert Gates, delivered more bad news. America’s shell-shocked troops in Iraq and Afghanistan would have their 12-month tours stretched out to 16 months. It was not long before Casey began calling the relentless rotation of troops in and out of combat zones the “new normal.”

While Casey has never been an alarmist or played on the nation’s fears, he foresees a persistent campaign against violent extremist groups like al-Qaeda. As he sees it, the Army must be prepared to deal with a full “spectrum” of potential conflicts. For the future not only holds the danger of terrorism and the rise of failed states, but also struggles to gain control of diminishing resources — energy, food, water — made more acute by a new status quo.

The nuclear dangers of Iran, Pakistan and North Korea loom large, but natural resources and mineral wealth have long been a spigot of state power. In the months leading up to World War II, the U.S. and several other nations used an oil embargo to pressure Japan into diplomatic concessions. They received no concessions and Japan ultimately responded with an attack on Pearl Harbor.

In 2006 and 2009, Russia cut off gas supplies in a trade dispute with Ukraine. That threatened one-quarter of the natural gas consumed by fuel hungry Europe, large parts of which fall under the protective arm of the North Atlantic Treaty Organization.

Still, there are “people in the Pentagon who say that ‘persistent conflict’ does not necessarily mean ‘persistent combat,’” says Nathan Freier, a senior fellow at the Center for Strategic and International Studies. “It is unlikely that the U.S. will be participating in large-scale serial wars.”

Freier had been part of the Pentagon team that developed the concept of persistent conflict. “There are people who are going to push back against U.S. influence around the world for a variety of motivations,” he says. “Conflict is not necessarily violent. We were thinking along the lines of ‘persistent conflict and resistance,’ a kind of push-back — economic, political and advisory responses other than combat.”

Whatever “persistent conflict” comes to mean, the Army is building what it calls a more modular force, one that allows specialized units to mix and match capabilities. The Army that will emerge from Iraq or Afghanistan — whether it finally leaves or stays behind in the name of stability — will have to be flexible enough to face an unpredictable future.

Professor Richard H. Kohn studies the connections between the military and American society at the University of North Carolina at Chapel Hill. “The term [persistent conflict] serves to explain why General Casey is moving toward an expeditionary army,” meaning a force that can be readily inserted into a conflict, Kohn says. A recent release from the U.S. Army War College’s electronic newsletter described the move to a modular Army as momentous: “[The Army's] largest transformation since World War II.” That means the great land army will no longer rely on massive divisions to enter combat, but rely on smaller Lego-styled brigades that can be shaped to the circumstances. Speaking at the annual war college strategy conference in 2010, (see video here) retired Army Lt. Col. Nathan Frier said, “Once, the gold standard of risk assessment for DOD was the ability to conduct two major war simultaneously, it is now the ability to simultaneously conduct dissimilar missions of widely varying scope and intensity.”

In early April 2011, U.S. forces were conducting dissimilar missions: The Navy was launching missiles and aircraft against Libya, while special forces units ran covert operations on the ground; the Army was shipping out and closing down bases in Iraq, while preparing to leave behind 150 uniformed personnel to work with security contractors to train Iraqi police and military; troops in Afghanistan were gearing up for a spring offensive against the Taliban (in a combat operation that had initially been aimed at al-Qaeda); elsewhere the Navy was off the coast of Japan performing humanitarian services while, at the same time, watching over its bases there.

The War College’s strategic conference also broached the problem of “indecisive wars,” which might be described as the residual effect of persistent conflict. Such wars would not be aimed at winning, but at containing or controlling a threat. While the Army has fought indecisive wars in the past, it was seldom by plan. If Kohn is correct, the new Army must be prepared to face up to this new reality: engaging in warfare that may not have a final outcome; war in which there will be no walk-off victory.

Another clear message emerging after 15 years of persistent conflict, beginning in Bosnia, is that troops wear out. Humans cannot simply be refitted and sent back into battle. The Army Times reports that the Department of Veterans Affairs has found that 44 percent of Iraq-Afghanistan war vets seeking treatment suffer war-related mental problems.

These numbers are only a slice of the real problem. Though the VA says it has treated a total of 178,483 individual veterans from 2002 to 2008 with war-related mental problems, or 24 percent of the 1.7 million men and women who have been in combat zones, a separate study by the RAND Corporation found that many soldiers don’t seek help for fear of repercussions. So the actual percentage of mental problems could be far higher.

Dr. Andrew Bacevich, a Vietnam veteran and the author of Washington Rules: America’s Path to Permanent War, is alarmed. “There is emerging evidence that shows that we’ve got a new generation of veterans that is going to have all kinds of problems. And that’s going to cost a lot of money. I believe it’s true that 40 percent of Iraq and Afghanistan veterans who have left the service have already filed for some kind of disability with the Veterans Administration, and that’s a huge number. And it’s a much large percentage than it was in previous wars.”

And if the soldiers are wearing out, so is their weaponry. None of the military’s gear was designed for 10 years of constant use. Workhorse systems, like the venerable C-130 aircraft used in transport and modified to go into battle as gunships, are falling apart. Air Force Lt. Col. Brenda Cartier commanded a squadron of these aircraft and says “the wing-boxes [have worn out] and need to be replaced.” Wing-boxes keep the wings attached to the fuselage.

In the end, the meaning of “persistent conflict” may ultimately be defined by the federal budget. The January/February issue of the journal Foreign Affairs had this: “The United States faces a watershed moment: It must decide whether to increase its already massive debt in order to continue being the world’s sheriff, or restrain its military missions and focus on economic recovery. …”

The authors of the article, Matthew Leatherman, a research associate for Washington’s Stimson Center, and Gordon Adams, a professor at American University and the former associate director for national security and international affairs at the Office of Management and Budget, see Pentagon spending in retreat.

“Secretary Gates is engaged in budgetary trench warfare,” Adams says. “He’s basically retreating one trench at a time, conceding that we’re at a turning point.”

Adams, who spent five years analyzing defense dollars in the Clinton administration, dismisses the concept of persistent conflict as a “fantasy.” He argues that the Defense Department has drawn the wrong lesson from Iraq and Afghanistan. The reality is that persistent conflict cannot mean persistent combat. It is both physically and fiscally impossible.

“We’re not going to fight insurgencies forever,” Adams says. “Casey has built a myth around Iraq and Afghanistan, saying that there will be an endless series of such wars that require a very large army.” Adams believes that the real lesson is more ironic: that when you invade a country, even a failed state, “you create your own insurgency.” His comments suggest that “persistent conflict” may become a self-fulfilling prophesy.

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