On Thursday, Defense Secretary Robert Gates announced the next stage of the most important military invasion of the 21st century. It isn’t the invasion of Iraq, Afghanistan, or Libya. It’s the invasion of warfare by unmanned vehicles.

The invasion began quietly years ago, with scattered, occasional reports of drone strikes in Pakistan. As these reports accumulated, it gradually became apparent that the U.S., without putting troops on the ground or sending pilots into Pakistani air space, was using drones to wage the world’s first remote-controlled war. That was the invasion’s first stage.

The drone campaign began as President Bush’s war. Then, with President Obama’s election, it crossed the political aisle. The rate of drone strikes tripled, and Bush’s war became Obama’s. (On Friday, a U.S. drone killed another 23 people in Pakistan.) That was the second stage.

Drones were deployed to Afghanistan and Iraq, but only as adjuncts to U.S. air and ground forces. Only in Pakistan did we wage a fully remote-controlled war—until Thursday. That’s when Gates and Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, announced another American drone campaign, this time in Libya.

This is the invasion’s third stage: global expansion. Pakistan has been a pilot experiment—or, rather, a remotely piloted experiment—in unmanned warfare. Drones have won the confidence of presidents of both parties. Gates’ announcement signals that they will now be deployed beyond Pakistan, to Libya and any other place where we need to kill people without risking American lives.

The quiet, early days of the drone war in Pakistan are over. Unmanned aerial weapons have become an American boast. “Gates: Obama OKs Predator Strikes in Libya,” says the headline on the Department of Defense website. The arrival of our killing machines is now part of the U.S. message to Muammar Qaddafi, the people around him, and our allies.

Why are we sending drones a month after we entered the Libyan war? Because the war has evolved to require them. Thanks to NATO’s air campaign, Cartwright explained, Qaddafi’s forces “that are out in the open know that they’re going to probably perish if a NATO bird sees them. So you’re seeing a much more dispersed fight, people that are digging in or nestling up against crowded areas, where collateral damage is.” To evade or deter air strikes, Qaddafi’s men are traveling in unmarked vehicles and relocating to cities where they can use nearby civilians, in effect, as human shields.

To kill the bad guys without killing innocent bystanders, we need vehicles that can get close enough to our targets—and inspect them long enough—to be sure that what we’re looking at is the bad guys. And then we have to hit them with weapons precise enough to avoid collateral damage. Drones have proved they can do this. Even critics concede that in Pakistan, the drones’ civilian casualty rate has declined from 25 percent to 5 percent.

In Libya, Cartwright observed, drones will give NATO the “ability to get down lower” for “better visibility” on its targets. “They’re uniquely suited for … urban areas where you can get low collateral damage,” thanks to “their extended persistence on the target.” Any pilot who tried to fly low enough, or hover long enough, to get the same level of visual confirmation might be shot down. And we can’t have that, because Obama has promised us an almost risk-free war.

On Thursday, Gates reaffirmed the pledge with which Obama began the Libya campaign: no ground troops. When reporters asked whether the drones’ arrival signaled “mission creep” in Libya, Gates said no. “The president has been firm, for example, on boots on the ground,” Gates reiterated. With the drones’ help, Obama intends to keep that pledge, waging a war without footprints. He won’t even have to risk another downed American pilot.

Drones alone can’t win the war in Libya, any more than they’ve won the war in Pakistan. But they increase our ability to kill the enemy while sparing civilians and avoiding risk to ourselves. To that extent, the unmanned invasion of warfare is a force for good.

On the other hand, it may also create a new kind of mission creep.

“If we tried to overthrow Qaddafi,” Obama warned Americans three weeks ago, “we would likely have to put U.S. troops on the ground to accomplish that mission, or risk killing many civilians from the air. The dangers faced by our men and women in uniform would be far greater.”

But if drones continue to improve and to take over the conduct of war, the risks to civilians, U.S. troops, and pilots might diminish to the point where we feel emboldened to attempt the overthrow of other dictators. In that case, the unmanned invasion of warfare might turn out to be the most significant invasion of this century, but certainly not the last.

(Readings I recommend: Spencer Ackerman at Danger Room points out that drones need spotters on the ground, so if we don’t use “boots” for that in Libya, we might be using the CIA. Peter Bergen and Katherine Tiedemann analyzed the first six years of the drone campaign in Pakistan in a 2010 New America Foundation paper. They updated their assessment four months ago in Foreign Policy. Bill Roggio and Alexander Mayer calculate a lower rate of civilian casualties at the Long War Journal. P. W. Singer wrote a terrific overview of drone warfare and the future of unmanned systems in Slate last year.)

Like Slate on Facebook. Follow us on Twitter. Human Nature’s latest short takes on the news, via Twitter: