Whenever someone proposes responding to a crisis with sanctions, the first response is that sanctions don't work. Then there's a flurry of media stories asking the question "Do sanctions work?" or "Can sanctions work in [country x]?".
There are three good reasons why we should break this cycle.
First, sanctions often aren't designed to succeed (ungated). They're sometimes designed to appease those who demand that the US "do something" when the president doesn't actually care enough about the issue to threaten force. They may even be least likely to be chosen when they'd be most likely to succeed. For sanctions to work, they need to have actual bite. They need to come from the right sources (ungated), and they need to be strictly enforced. Enforcing sanctions is not easy, since many industries within the sanctioning state would prefer to go right on doing business (see here and here). The failure of sanctions that were were not designed in such a way as to give them their best chance of success obviously biases our estimate of how well they could work.
Second, the mere threat of sanctions often is successful. When we observe sanction imposition, we're already looking at a truncated sample that includes the most resolved targets. If we observe the low rate of concessions being granted subsequent to sanctions being imposed and infer from this that sanctions are an ineffective tool of statecraft, we'll inappropriately be reluctant to even threaten using them in the first place, and thus forego opportunities to extract easy concessions. Granted, this point doesn't necessarily speak to the current debate about what to do about Iran (which I'll return to below), but it's still worth remembering that we shouldn't be too quick to dismiss economic coercion.
Third, and, most importantly -- no one ever asks "does military coercion work?" Yet the (sometimes) unstated conclusion of most "sanctions don't work" arguments is "therefore we should use force."
You may find it odd that I'd even raise the question of whether military force works. Of course it does, you're thinking. And you're ready to rattle off a set of carefully chosen, yet hardly representative, examples of success.
But bear with me for just a moment.
There are two ways that the application of military force might succeed.* The first is by brute force. If the goal is to conquer territory, repel an enemy invasion, destroy weapons factories etc, then the goal can be accomplished directly through military force.
The second is through coercion. Here, the goal is to inflict enough pain on the opponent to induce voluntary compliance. The logic here is precisely identical to that of sanctions. There is no theoretical difference between the two. In practice, it may be either easier or more cost effective to apply the level of pressure necessary to ensure compliance through military force. In others, it won't be. It depends on the circumstances.
Thus it ought not to surprise us that militarily powerful states tend to achieve their objectives when said objectives can be accomplished through brute force, but in fact have a stunningly poor track record when it comes to using military force to ensure voluntary compliance. Compare "defend regime" and "remove regime" to "peacekeeping" and, most importantly, "policy change", in the figure below (taken from this excellent article by Patricia Sullivan):
Consider the current standoff between the US and Iran. In principle, it's possible to prevent a state from acquiring WMD through brute force alone. Ask the Israelis, who did so in Iraq and (probably) Syria. But many experts believe that will be difficult to impossible with Iran. So if the US and/or Israel are going to stop Iran from acquiring the bomb, they're going to be attempting to achieve a goal of policy change through coercion. And coercion, though possible, is not easy -- whether you're attempting it through sanctions or by military force. Especially when the goal is policy change.
It's not at all clear to me that Iran is in fact going to develop a nuclear weapon. And it's not clear to me that if they did, this would be an outcome the US and Israel could not live with. (The Isrealis seem to be coming around to that idea too.) But I'll leave it to others to sort out what price the US and/or Israel is, or should be, willing to pay in order to coerce Iran. It's not my job to make those kinds of value judgments. What I can tell those of you who are convinced that "something must be done" is that neither our theoretical understanding of coercive bargaining nor the historical record provides much support for the belief that military force will "work" against Iran whilst sanctions will not.
*Regarding the question of threats to use military force, see here and here.