It's easy to take American military invincibility for granted. On land, at sea and in the air, no force can match us -- a situation that has prevailed since the Soviet Union collapsed.

But we have to avoid complacency and remain vigilant. We must be especially wary about protecting the final frontier: space. Falling behind there would jeopardize all our other forces.

After all, our planes, ships and even warriors on the ground depend on precise information from our observation and communications satellites. If we lose our eyes in the sky, we could find ourselves losing battles on the Earth.

This could happen swiftly. Recently, China used a ground-based ballistic missile to destroy one of its own obsolete weather satellites -- proof that Beijing is perfecting weapons able to wipe out orbiting objects.

A spokesman for China's foreign ministry claims his country remains committed to the "peaceful development of outer space." But it's difficult to see how blowing things up promotes peace.

Luckily, this isn't 1957, and the Chinese test wasn't Sputnik; it didn't catch us by surprise.

The United States already has a national space policy in place. Issued by President Bush last summer, it has many solid points. For example, the policy correctly insists the United States must "take those actions necessary to protect its space capabilities; respond to interference; and deny, if necessary, adversaries the use of space capabilities hostile to U.S. national interests."

To do that, though, we must recognize one key fact: There are already weapons in space.

Arms control advocates disagree, of course. They frequently claim the Bush administration is planning to take provocative and irresponsible steps to introduce weapons into space. For example, one article written by Jeffrey Lewis of the Center for Defense Information is titled "What if space were weaponized."

But weapons have been passing through space for decades, specifically since World War II, when German V-2 rockets were fired into space on their way to targets in Britain.

Today, our military (among others, of course) uses satellites to monitor friend and foe and to guide forces around the globe.

The real threat is that America will fall prey to appeals to "disarm" space just as others, such as the Chinese, are increasing their presence there. In 2005, Ohio Rep. Dennis Kucinich, a once and future Democratic presidential candidate, tried to pass a measure that would have required President Bush to negotiate a treaty "banning space-based weapons."

Since Kucinich left the definition of "space-based weapons" vague, such a treaty could conceivably have required the United States to withdraw all our navigation, communications and command-and-control satellites. We use those critical resources to identify enemy targets and direct U.S. weapons against them. We can't afford to lose those satellites, let alone give them up voluntarily.

Of course, space is far off, so it's easy to forget that threats based there can affect our daily lives. But Bush's space policy directive gives our government the ability to alert Americans to potential threats by allowing the secretary of defense and the director of national intelligence to classify and declassify information on national security activities in space.

To ensure that Americans understand the threats we may face, Defense Secretary Robert Gates and Director of National Intelligence Mike McConnell should act quickly to release as much information as they can about China's policies and actions in space.

As long as Americans understand what's going on above them, they'll demand that their elected representatives protect them from space-based threats. After all, preserving American assets in space is the only way to ensure we can continue building a peaceful future down here on Earth.

Edwin Feulner is president of The Heritage Foundation (heritage.org), a Washington-based public policy research institute and co-author of the new book Getting America Right.