I wouldn’t have believed you if you’d told me 20 years ago that America would someday be routinely firing missiles into countries it’s not at war with. For that matter, I wouldn’t have believed you if you’d told me a few months ago that America would soon be plotting the assassination of an American citizen who lives abroad.

Shows you how much I know. President Obama, who during his first year in office oversaw more drone strikes in Pakistan than occurred during the entire Bush presidency, last week surpassed his predecessor in a second respect: he authorized the assassination of an American — Anwar al-Awlaki, the radical Imam who after 9/11 moved from Virginia to Yemen, a base from which he inspires such people as the Fort Hood shooter and the would-be underwear bomber.

Students of the law might raise a couple of questions: 1) Doesn’t it violate international law to fire missiles into Pakistan (especially on a roughly weekly basis) when the Pakistani government has given no formal authorization? 2) Wouldn’t firing a missile at al-Awlaki in Yemen compound the international-law question with a constitutional question — namely whether giving the death penalty to an American without judicially establishing his guilt deprives him of due process?

I’m not qualified to answer these questions, and, besides, it doesn’t really matter what the correct answers are. The Obama administration has its lawyers scurrying to convince us that the answers are no and no, somewhat as the Bush administration dispatched John Yoo to justify its torture policy. And these answers, regardless of their legal merit, will be accepted so long as Americans are convinced that being safe in the post-9/11 world requires accepting them.

So maybe the question to ask is whether Americans should be convinced of that — whether assassinating terrorists really helps keep us safe.

There’s no way of answering this question with complete confidence, but it turns out there are some relevant and little-known data. They were compiled by Jenna Jordan of the University of Chicago, who published her findings last year in the journal Security Studies. She studied 298 attempts, from 1945 through 2004, to weaken or eliminate terrorist groups through “leadership decapitation” — eliminating people in senior positions.

Her work suggests that decapitation doesn’t lower the life expectancy of the decapitated groups — and, if anything, may have the opposite effect.

Particularly ominous are Jordan’s findings about groups that, like Al Qaeda and the Taliban, are religious. The chances that a religious terrorist group will collapse in the wake of a decapitation strategy are 17 percent. Of course, that’s better than zero, but it turns out that the chances of such a group fading away when there’s no decapitation are 33 percent. In other words, killing leaders of a religious terrorist group seems to increase the group’s chances of survival from 67 percent to 83 percent.

Of course the usual caveat applies: It’s hard to disentangle cause and effect. Maybe it’s the more formidable terrorist groups that invite decapitation in the first place — and, needless to say, formidable groups are good at survival. Still, the other interpretation of Jordan’s findings — that decapitation just doesn’t work, and in some cases is counterproductive — does make sense when you think about it.

For starters, reflect on your personal workplace experience. When an executive leaves a company — whether through retirement, relocation or death — what happens? Exactly: He or she gets replaced. And about half the time (in my experience, at least) the successor is more capable than the predecessor. There’s no reason to think things would work differently in a terrorist organization.

Maybe that’s why newspapers keep reporting the death of a “high ranking Al  Qaeda lieutenant”; it isn’t that we keep killing the same guy, but rather that there’s an endless stream of replacements. You’re not going to end the terrorism business by putting individual terrorists out of business.

You might as well try to end the personal computer business by killing executives at Apple and Dell. Capitalism being the stubborn thing it is, new executives would fill the void, so long as there was a demand for computers.

Of course, if you did enough killing, you might make the job of computer executive so unattractive that companies had to pay more and more for ever-less-capable executives. But that’s one difference between the computer business and the terrorism business. Terrorists aren’t in it for the money to begin with. They have less tangible incentives — and some of these may be strengthened by targeted killings.

One of the main incentives is a kind of local prestige grounded in grievance. When people feel aggrieved — feel that foreigners have wronged them or exploited them or disrespected them — they may admire and appreciate those who fight on their behalf. Terrorists are nourished ultimately by a grass-roots sense of injustice.

And one good way to stoke a sense of injustice is to fire missiles into cars, homes and offices in hopes of killing terrorists, while in fact killing no few innocent civilians. Estimates of the ratio of civilians to militants killed are all over the map — 50 to 1 or 10 to 1 or 1 to 2 or 1 to 10 — but the estimate of the Pakistani people, which is all that matters, tends toward the higher end. And the notion that these strikes are a kind of national humiliation long ago entered Pakistani culture. A popular song from a couple of years ago says Americans “kill people like insects.”

You can imagine why, as Jordan’s data suggest, this counterproductive effect of decapitation might be stronger for religious groups than for groups driven by a secular ideology. To the intensely religious even the harshest adversity can seem like a test administered by a God who will reward faithful perseverance. And the belief that death in a holy war gets you to heaven can’t hurt when you’re looking for someone to replace an assassinated leader.

Obviously, drone strikes must do some short-term good by disrupting the operations of Al Qaeda or the Taliban. And there’s no way of knowing for sure that this short-term good is outweighed by a long-term recruiting boost. But Jordan’s data, combined with common-sense analysis, make the merits of our current strategy far from clear.

By itself that wouldn’t be a damning indictment. You have to choose some strategy in the face of uncertainty, and if this turns out to be the wrong one — well, that’s life; strategic failure happens.

But in this case the price we pay goes far beyond failure. If Harold Koh — the state department lawyer assigned the job of justifying Obama’s strategy — carries the day, America will be telling the world that it’s O.K. to lob missiles into countries that haven’t attacked you, as long as you think a terrorist may live there. Do we really want to send that message to, for example, Russia and China, both of which have terrorism problems? Or India or Pakistan?

And are we sure we want to say that, actually, due process of law isn’t really guaranteed all American citizens so long as there’s a war on terrorism — which, remember, is a war that may continue for eternity?

I’m not sure I’d want to pay these prices even for a strategy that helped us in the war against terrorism. To pay them for a strategy that may be hurting us is even less appetizing.

Note: Read Gary Hart’s response to this column.