A historic change is underway in the global security system. As Harvard political scientist Stephen Walt wrote, the world is witnessing "a sharp decline in America's ability to shape the global order." In the future, Walt and others believe, "the United States simply won't have the resources to devote to international affairs that it had in the past." Christopher Layne of the George Bush School of Government and Public Service at Texas A&M University is even more blunt: "The epoch of American dominance is drawing to a close, and international politics is entering a period of transition: no longer unipolar but not yet fully multipolar."
The trends are clear—no one doubts that the U.S. global presence is receding. But no one knows how far and how fast this will go or, importantly, what it will mean. This is especially true in the military realm.
To a large extent, the type of armed forces that the United States elects to maintain will affect both the role Washington can play in the future global security system and the way the world responds to declining American involvement. The United States did not always have this degree of strategic choice: The modern U.S. military was largely built in response to the capabilities of America's enemies, initially Nazi Germany and Imperial Japan and subsequently the Soviet Union and its clients. Defeating or deterring these enemies required an American military with balanced capabilities including air, sea and land power, and conventional and unconventional forces. Because the United States had to project power over long distances, it needed a dominant navy and air force. But it also needed the ability to undertake lengthy conventional wars and stabilization or peacekeeping campaigns, which required robust land power. A multidimensional, balanced military of this sort enabled the United States to help manage security in diverse regions and against diverse threats.