By James Jay Carafano

America's military is shrinking - and not by accident.

Over the last five years, Washington has slashed defense spending by almost 25 percent. Whether measured as a percentage of gross domestic product or as a portion of the federal budget, the Pentagon's operating budget is near historic lows for the modern era - and is slated to go lower.

Shrinking funds necessitate shrinking forces. Critics decry that today we're defended by the smallest arsenal of ships, planes, and warriors since before World War II. That's an interesting factoid but not a compelling reason to rebuild the military.

The real issue is: How much military do we really need?

The Heritage Foundation will answer that question later this month with the publication of its third annual Index of U.S. Military Strength. It's a comprehensive, objective review of not just the fighting capabilities of our armed forces but also the threats they must be prepared to deal with and the environments in which they will have to operate.

And it gives Americans the information they need to judge for themselves whether the Pentagon has what it needs to preserve freedom, protect our national interests abroad, and keep us safe.

Measuring the military starts with answering the question: Safe from what?

The index lays out three core missions for the armed forces.

The first is safeguarding the homeland from attack. Enough said.

The second mission is ensuring the freedom of the commons. In other words, preventing others from closing the critical routes of commerce, travel, and communication. Assuring freedom in these commons, owned by no nation, allows America and other nations to make their place in the world.

The third vital mission is preventing major regional conflicts in Europe, the Middle East, and Asia. A big war in any of these places could quickly become a very dangerous problem for us.

Objectively grading the military's capacity to fulfill these missions requires more than just adding up troop and equipment numbers and studying readiness assessments. You also have to assess the nature and acuteness of the threats in each key region.

Other factors - such as what our allies can contribute and the operating environment (the geography and infrastructure in each region) - go into determining our military strength.

For three years running, this type of assessment has revealed U.S. military strength to be, at best, marginally capable of executing its three core missions successfully.

Our vital national interests are at significant risk, and shrinking the military further will only make matters worse.

Our enemies have been happy to see our global influence and national defense waste away. It's time for Washington to let them know that playtime is over. And that means rebuilding our military.

To bolster the resolve of NATO and contribute to the stability of Europe, the buildup must include ground and air forces. Our naval forces must be strengthened too, sending a clear message that China won't be pushing us out of Asia or the Pacific anytime soon.

And we need to build up our missile defenses, so Iran and North Korea will have to abandon their dreams of being able to threaten others with nuclear holocaust. Other work needs doing as well.

The sooner the buildup starts, the sooner our adversaries around the world will get the message that America is back and once more ready, able, and willing to defend its legitimate interests around the globe.

That's not say we don't need allies to do more or statecraft to convince bad people to stop doing bad things. But those efforts are most successful when there is a military to back them up. Peace through strength is an apt mantra, especially in these increasingly troubled times.

A 25-year Army veteran, James Jay Carafano is a vice president at the Heritage Foundation (www.heritage.org).