Yesterday, Congress overwhelmingly—in the Senate, the vote was 99-0—approved new sanctions against Iran intended to punish the country for its pursuit of nuclear weapons in violation of the Nuclear Non-Proliferation Treaty. The move follows the United Nations Security Council’s decision this week to impose what President Obama calls “the toughest sanctions ever faced by the Iranian government.” Our sanctions—which go a little farther than the U.N.’s—target banks and companies that either sell Iran energy products like refined gasoline to Iran, provide it with technology it can use to restrict the free flow of information, or that do business with the Iranian Revolutionary Guard. And everyone in Congress seems to agree these sanctions are a great idea.

That’s precisely what makes me nervous. Maybe I'm too cynical. But whenever I hear that a bill has passed unopposed I wonder what is wrong with it. My first impulse is to assume that it is a poorly-conceived political stunt. If there were any serious debate over the bill, there would inevitably have been some opposition. When a bill passes unanimously, we can be sure that politicians either think they can score easy political points by supporting it or are worried about how opposing it would look.

Most Americans will probably agree that Iran—which continues to sponsor terrorism and has repeatedly threatened to wipe Israel off the map—is not a country we want to have nuclear weapons. Iranian President Mahmoud Ahmedinajad is in power thanks to what appears to have been a fixed election, after quashing largely peaceful popular protests with the help of the Revolutionary Guard.

But sanctions have mixed record when it comes to changing countries’ behavior. Targeted, “smart” sanctions like the ones against Iran have the advantage that they easier to swallow and are less likely to hurt Iranians citizens with little control over their government’s policy. But they're not necessarily very effective. And they often complicate diplomats' efforts to strike deals with those elements of foreign governments that are willing to work with us. They're especially problematic in a region like the Middle East, where we're widely seen as throwing our weight around too much already. Indeed, they may even backfire, if they convince Iranians that the only way to avoid being bullied by us is to develop a nuclear deterrent.

In fact, as Christopher Wall argues in Foreign Policy, these sanctions aren’t likely to have much effect. They're probably as much a media event as anything else. Daniel Drezner has it exactly right when he says that sometimes the purpose of sanctions like these is “to lull policymakers into believing that they're doing something when they're not.” Congress can tell Americans that having passed these sanctions it has done what it could. But a handful of sanctions are no substitute for the real work of meaningful diplomacy.