President Barack Obama's administration is considering a ban on monitoring the communications of allied heads of state, following reports that the National Security Agency tapped the mobile phone of German Chancellor Angela Merkel for 10 years. That's a fine idea, except that recent history suggests it won't last.

The U.S. has a long history of imposing unilateral restrictions on its intelligence community, only to later weaken or reverse those restrictions. The Cipher Bureau, an early predecessor to the NSA created during World War I, was shuttered in 1929 by Secretary of State Henry Stimson, who famously and punctiliously proclaimed that "Gentlemen do not read each other's mail."

The intention may have been noble, but the effect was short-lived. The U.S. Army responded by creating the Signal Intelligence Service. Today, Stimson's concerns seem quaint.

The U.S. has also tried sanitizing its approach to human intelligence, with those changes also failing to stick. In 1995, after reports that the Central Intelligence Agency had recruited agents in Guatemala who took part in human-rights abuses, Director John Deutch reportedly banned the recruitment of so-called unsavory assets.

After the Sept. 11 attacks, former officials roundly criticized the rule. "It's kind of a dirty business, and you have to deal with a lot of unsavory people," said former President and CIA Director George H.W. Bush. The agency eventually loosened the rule, and publicly said it had never quite existed.

The most telling example of unilateral bans that don't quite stick may be a series of presidential decrees prohibiting assassinations. In 1976, after allegations that the CIA had killed foreign leaders, President Gerald Ford signed Executive Order 11905, which said "No employee of the United States Government shall engage in, or conspire to engage in, political assassination."

Presidents Jimmy Carter issued a similar order, and President Ronald Reagan expanded it, decreeing in 1981 simply that "No person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, assassination."

The order didn't quite work as planned. One effect was to turn what might have been assassination attempts into military strikes that had a good chance to killing foreign leaders -- what might be called the "no tears" exemption. When Reagan ordered the bombing of Muammar Qaddafi's compound in 1986, he said that the point wasn't to kill Qaddafi, but "I don't think any of us would have shed any tears if that happened." The journalist Daniel Schorrcalled it "presidential double-talk."

Even that exemption proved insufficiently accommodating. President Bill Clinton interpreted the ban to exclude the leaders of terrorist organizations, such as Osama bin Laden. And after Sept. 11, President George W. Bush decided that the ban on assassinations also didn't apply to terrorists of lesser importance. Thanks to that interpretation, an estimated 2,200 people have been killed by U.S. drone strikes in Pakistan alone over the past decade.

In other words, the prohibition on assassinations doesn't seem to have imposed significant limitations on the U.S.'s ability to kill the people it views as a threat, whatever one wants to call it. That doesn't make American policy hypocritical; it just suggests each president interprets the restrictions imposed by previous administrations in a way he thinks will meet the country's present security needs.

In fact, when I asked two foreign-policy professionals to suggest a self-imposed restriction on U.S. intelligence that has some staying power, the only one they could think of was the prohibition on former CIA employees working for the Peace Corps. (Am I missing any other examples? Tweet them to me.)

Would a ban against spying on the leaders of ally states be any more immune to workarounds? Perhaps. But a more likely scenario is that future administrations would find ways around it -- and, if they didn't, would find a way to rescind it.

(Christopher Flavelle is a member of Bloomberg View's editorial board. Follow him on Twitter at @cflav.)