America is a great nation. It enjoys unprecedented wealth. Its people are among the freest in the world. And, with the world's most powerful armed forces, the United States is largely the master of its own destiny.

Yet there is an insidious threat to America's continued greatness. It gets scarce attention because its effects are not immediate, but that threat is real. It is the threat of crushing government debt.

Few things can march a great power down the road of decline faster than irresponsible economic policies -- and huge debt is most often the drum leader of the pack. To learn which nations are most at the mercies of their enemies, just look at the World Bank's list of most "highly indebted poor countries."

Today, the federal government is piling up debt as never before. Publicly-held debt now exceeds $7 trillion -- about $22,000 per person. And it's expected to increase by $9 trillion over the next 10 years.

Yes, we had extremely high debt after World War II, but unlike today's debt, it was temporary. Today's massive and growing entitlement costs mean that, absent reform, the picture will only get worse over the long-term.

What difference does this make for U.S. security? The more we have to pay to service the rising debt and pay out for entitlements, the less there is for defense.

Unless we reverse course, this means that -- in our children's lifetime -- the U.S. military might be unable to protect a sea lane vital to trade and military supply lines. We might be unable to suppress an enemy regime that launches a terrorist attack against us. And absent the great American economic engine, we might lack the resources to stay on the cutting edge of technology, leaving our soldiers vulnerable to being matched or even trumped on the battlefield by better-equipped foes.

Other ruinous scenarios are possible as well. Suppose foreigners refuse to buy U.S. debt, leading to debt-induced inflation that eats away at productivity and the currency. In this situation, Americans could completely lose confidence in protecting our interests abroad. Beset by inflation and other towering economic problems, Americans could understandably turn toward isolationism, as they had by the 1930s. This would create power vacuums around the world and unleash enemies bent on challenging the great "sick man of America."

It has happened before. A great many nations have walked the slow road of decline, and in almost all cases -- from the Roman Empire, to Louis XIV's France, to the British Empire -- huge debt played a crucial role.

In those cases, decline was ushered in by the expense of large armies or grand building excesses like the Palace of Versailles. But an American decline would have a different root: runaway social entitlement spending. That's the main cause of our massive debt.

A descent down this path of decline is not inevitable, but it is a path that may be chosen freely by a people unable to understand the relationship between government debt and the means required to preserve America's unique position in the world.

No other country on earth can protect those sea lanes or keep a hegemonic power from dominating a continent. No other country can destroy a terrorist-haven regime. And no other country can field a strategic missile defense system to protect us and our allies.

Should Spain or Germany be attacked like we were on 9/11, they would have only two options: call Washington for help, or file a complaint with the United Nations.

Americans should never want to be in such a position. For Americans, our nation's great power status is not an option; it's a necessity. There is no other great power to bail us out if we get into trouble.

If you think this is an exaggeration, think again. The Congressional Budget Office estimates that just 10 years from now, the president's budget would nearly double our national debt, to more than 82 percent of what the economy produces every year. Interest payments on this debt alone will amount to $100 billion more than the entire budget for the Defense Department. Just imagine what those numbers will be by 2035.

Now imagine the pressure that will put on the defense budget. In 1965, mandatory spending consumed over a third of the federal budget. Today, it eats up approximately two-thirds. At this rate, if nothing changes, in just 19 years there will be nothing left over in the discretionary budget -- and that's where we get funding for our armed forces.

In a very real sense, the safety of every American, and their freedom to vote and live their lives unmolested from foreign threats, depend on our country's sustained ability to project power against our enemies no matter where they are. It's truly the only thing that completely sets us apart from other nations.

Kim Holmes is vice president of foreign- and defense-policy studies at the Heritage Foundation and author of "Liberty's Best Hope: American Leadership for the 21st Century."