A prominent study by the Washington-based Peterson Institute for International Economics found that sanctions attained their goal only in one-third of all cases. Yet the United Nations, the United States and the European Union recurrently impose sanctions. Indeed, their use of these coercive measures has gone up considerably since the end of the Cold War.
Sanctions are clearly a tricky tactic, but there are conditions under which they can be useful. First of all, it's advantageous if the imposing and targeted countries have strong economic, political and social ties. It's also helpful if the targeted country is well-integrated in the global economy, because this means it will be more susceptible to outside pressure. In 2012 the European Union - one of Iran's most important customers - imposed a wide-ranging oil embargo on the country. This hurt Iran's economy more than any other previous measure.
Democracies are particularly vulnerable to economic sanctions because citizens more easily react in protest against their governments. The problem, of course, is that it's mostly authoritarian regimes violating human rights, pursuing nuclear weapons programs or supporting terrorists. This is why it helps, in many cases, to have a broad coalition in support of sanctions. Such a united front also helps to prevent the occurrence of “black knights,” countries not involved in sanctions that swoop in to help the targeted country, countering efforts to exert pressure on, say, Belarus, Zimbabwe or Myanmar. The flip side is that broad coalitions often settle for very weak measures to find common ground. Still, the signal can be very powerful, as was the case in South Africa in the final days of Apartheid rule, when a lot of white South Africans and business leaders simply grew tired of being isolated.
However, high economic costs don't necessarily translate into meaningful political pressure all the time. Often, results can be quite the opposite; the target feels under siege and closes ranks to resist what is recurrently called “imperialist” pressure from the outside. In inward-looking and highly repressive North Korea, leader Kim Jong-un is hardly bothered by the sanctions imposed on the country. The almost complete United Nations trade embargo imposed on Iraq in 1990 had devastating humanitarian consequences for the Iraqi people, but left dictator Saddam Hussein and his coterie largely untouched. It is against this background that the United Nations, the United States and the European Union now try to directly hit the ruling elites; to freeze their bank accounts, to stop trade with their companies and to impose travel bans on them.
The bottom line is that comprehensive sanctions can work, but often the poor and vulnerable will suffer. It's perhaps more productive in many cases, therefore, to target the wealthy and elite sectors of countries instead of the mass populations.