This is part of an ongoing series preparing for the AEI/CNN/Heritage National Security & Foreign Policy GOP presidential debate on November 22.

After World War II, after Korea, after Vietnam, and after the Cold War, Americans have naturally reduced the size of their armed forces and reduced levels of defense spending.  Preparing for retreat from Iraq and, soon after, in Afghanistan, oppressed by a sluggish economy and a metastasizing federal debt, the United States is laying plans to reduce its military to dangerously weak levels.

"What has made the United States exceptional is not just political principles that are universal in theory, but principles that guide the vigorous exercise of power." --Thomas Donnelly

The Army and Navy are on track to return to pre-World War I scale. The Air Force will be cut to a level perhaps without precedent in its history; a decade ago, “air power” was the signature form of American power, now we try to shape the world with “soft power.” Absent a reversal of course, the next commander-in-chief may not be able to preserve American global leadership. His only choice may be to “lead from behind.”

Seen in larger context, the reductions of the Obama years, the Budget Control Act and those that may emanate from the congressional “Super Committee” are a continuation of the extended Cold War defense drawdown. Almost an entire generation of new weaponry has been eliminated; U.S. forces fly planes, sail ships, and drive combat vehicles built in the Reagan years. Soon, the strength of active-duty forces will be half what it was in Desert Storm.

For conservatives, one of the tests of American virtue has been American strength.  What has made the United States exceptional is not just political principles that are universal in theory, but principles that guide the vigorous exercise of power. Thus, a candidate who would restore America’s greatness must also restore American military power.

Thomas Donnelly is a resident fellow and director of the Center for Defense Studies at AEI.