There are lots of reasons for excluding gays and lesbians from the military. But current supporters of the "don't ask, don't tell" policy insist that really, it all comes down to cohesion. Keep gays out, and soldiers will stick together through thick and thin. Let gays in, and every platoon will disintegrate like a sand castle in the surf.

John McCain sounded this theme at a Senate hearing the other day, arguing that the existing law rests on the belief "that the essence of military capability is good order and unit cohesion, and that any practice which puts those goals at unacceptable risk can be restricted." A group of retired military officers said the ban on gays serves "to protect unit cohesion and morale."

Maybe this concern is what really underlies the exclusion of gays and lesbians. But I'm not so sure. In 2007, Gen. Peter Pace, then chairman of the Joint Chiefs of Staff, was asked about it, and he offered a different rationale. "I believe homosexual acts between two individuals are immoral and that we should not condone immoral acts," he said. Could the opposition stem mostly from a simple aversion to gays and their ways?

It's not completely implausible that in a military environment, open homosexuality might wreak havoc on order and morale. But the striking thing about these claims is that they exist in a fact-free zone. From all the dire predictions, you would think a lifting of the ban would be an unprecedented leap into the dark, orchestrated by people who know nothing of the demands of military life.

As it happens, we now have a wealth of experience on which to evaluate the policy. When you examine it, you discover the reason McCain and Co. make a point of never mentioning it.

A couple of dozen countries already allow gays in uniform—including allies that have fought alongside our troops, such as Britain, Canada, and Australia. Just as there is plenty of opposition in the U.S. ranks, there was plenty of opposition when they changed their policies.

In Canada, 45 percent of service members said they would not work with gay colleagues, and a majority of British soldiers and sailors rejected the idea. There were warnings that hordes of military personnel would quit and promising youngsters would refuse to enlist.

But when the new day arrived, it turned out to be a big, fat non-event. The Canadian government reported "no effect." The British government observed "a marked lack of reaction." An Australian veterans group that opposed admitting gays later admitted that the services "have not had a lot of difficulty in this area."

Israel, being small, surrounded by hostile powers, and obsessed with security, can't afford to jeopardize its military strength for the sake of prissy ventures in political correctness. But its military not only accepts gays, it provides benefits to their same-sex partners, as it does with spouses. Has that policy sapped Israel's military might? Its enemies don't seem eager to test the proposition.

You could argue that none of these experiences is relevant, since, being Americans, we are utterly unique. But our soldiers don't seem to have any trouble fighting alongside gay soldiers from allied nations.

Not only that, but it turns out the U.S. military itself has tried the same policy with satisfactory results. Former Joint Chiefs Chairman Gen. John Shalikashvili has pointed out that "enforcement of the ban was suspended without problems during the Persian Gulf War, and there were no reports of angry departures."

That's right: We fought a war without the ban, and we won. In a pinch, our heterosexual men and women in uniform confirmed, they can function perfectly well amid openly gay colleagues.

That shouldn't be surprising, since the military requires its members to live with all sorts of people in close quarters and demanding conditions. A lot of recruits would be more leery of bunking next to an ex-con than a homosexual, but the military admits hundreds of felons each year, including some violent ones. If unit cohesion can survive the presence of killers, rapists, and child molesters, why would it shatter on contact with gays and lesbians?

All recent experience argues that the American military would adapt fine to accepting gays. But when it comes to actual real-world evidence, supporters of the ban don't ask, and they don't tell.