By Robert Burns/AP National Security Writer

The issue

Is the U.S. spending enough money on defense, and is it spending it in the right ways? In the aftermath of the 9/11 terrorist attacks the money spigot was turned wide open, pouring hundreds of billions of dollars into the wars in Iraq and Afghanistan and expanding the armed forces. Now that's changing, and an important issue in the election is whether budget cuts have gone too far.

Where they stand

President Barack Obama wants to put the brakes on growth in the defense budget. He reasons that a new era of austerity at the Pentagon won't hurt American security. Earlier this year, Obama adopted a military strategy to fit leaner budgets and to take into account that U.S. troops are winding down a decade of wars.

Republican nominee Mitt Romney takes a far different view. He argues that Obama has presided over a military decline, and defense budgets need to grow faster. He wants to add tens of billions of dollars a year to the Pentagon's core budget, particularly to build Navy ships at a faster pace and to reverse troop cuts.

Why it matters

There are plenty of potential security threats on the horizon, not to mention an unfinished war in Afghanistan.
The size and shape of the defense budget go a long way toward determining whether the U.S. can influence events abroad, prevent new wars and be ready for those it can't avoid. It also fuels the domestic defense industry in ways that affect the economic vitality of communities large and small across the country.

The Pentagon's budget, including war costs, is $670 billion this year, or about 18 percent of total federal spending. The dollar amount has more than doubled since 2001, when the U.S. began a decade of wars. Under Obama's plan it will grow by $259 billion less over the next five years than previously envisioned.

The hope is to align defense budgets with the most worrisome security threats, Iran being a case in point. Its suspected drive to build a nuclear weapon could lead to war. Diplomacy is at the forefront of Obama's strategy for preventing a nuclear-armed Iran, but at the same time he is building up U.S. forces in the Persian Gulf region near Iran, including missile defenses that could protect Gulf allies in the event of an Iran war.

Obama also wants more focus on Asia-Pacific security issues, starting with China's rapid military modernization. But that and other elements of his new military strategy could come apart at the seams if Congress and the administration don't come up with a way to avoid automatic budget cuts starting in January.

If such a budget deal is not reached in time, the Pentagon would absorb an additional $500 billion in cuts over the decade. That was put in place by a bipartisan deal reached in August 2011 between the White House and Congress.

At its core, the debate over whether the U.S. is spending enough on defense — and whether the dollars are being invested wisely — gets down to this: What should America defend against? Is it the al-Qaida terrorist network, which has been the central focus of U.S. defense strategy for a decade but is now in decline? Is it China, which is gaining military strength and flexing its muscle in regional disputes? Is it Russia, whose nuclear arsenal is the only one in the world still capable, in theory, of destroying the United States?

One lesson of 9/11 and the two wars that followed is that the U.S. has a poor track record of preparing for the right threat.
Why it Matters: Candidates have different plans for defense budgets