News came Monday that the Israeli navy had boarded a flotilla of ships seeking to run Israel’s blockade of Gaza, setting off an incident that ended with the Israelis shooting a number of people on board the ship. The immediate controversy is over what happened on board: the Israelis say they were attacked upon boarding by assailants wielding knives and clubs. Who you believe depends in large part where your sympathies lie in the longstanding struggle between Israel and its neighbors. But at bottom, incidents like Monday’s are the inevitable results of a policy of sanctions and the blockade required to enforce them. And advocates of such policies who claim that they are preferable to more openly aggressive means of dealing with security threats need to come to grips with that fact.

Israel, it is not news to report, has long suffered from serious security threats – suicide-terrorist attacks, rocket launches, and the movement of arms and hostile men that support such threats – emanating from Palestinian territory in Gaza and the West Bank. Some of Israel’s critics on the Left argue that Israel should have no response at all to these assaults on Israeli civilians, but plainly this is not a feasible option – such people would never argue that the United States should just ignore the spillage of oil in the Gulf of Mexico, much less people lobbing rockets into our territory. Nor is a law-enforcement-style manhunt of those responsible in the aftermath of attacks a sane solution, not least in light of the manifest impossibility of obtaining the cooperation of authorities in Gaza and the West Bank who range from personally complicit to indifferent to impotent.

Boiled down to essentials, there are three possible Israeli responses to such threats:

1) Retaliation in kind. Most everyone would agree that this is the worst of all options, the collective-punishment approach of targeting Palestinian civilians for death in response to any attack.

2) Invasion/occupation. Israel could return to a policy of entering and controlling Gaza, eliminating the state of semi-independence that allows threats to fester. This, too, is a strategy that is costly in lives and provocation.

3) Sanctions/blockade. Although it has not been the exclusive policy undertaken, imposing sanctions and a blockade on Gaza has been Israel’s main line of defense. Basically, the Israelis are trying to prevent the Palestinians from obtaining weapons – a policy of interdiction, similar to a gun-control or war-on-drugs approach, as opposed to going to the roots of the problem.

This isn’t the only purpose for which sanctions are used; in some cases, economic sanctions are used as retrospective punishments, or as leverage (put enough hurt on the other guy to make him cry uncle), or to deny a regime the resources to buy weapons. But the common thread is that any sanctions policy, to be effective, must be accompanied by enforcement of the sanctions through some form of blockade.

The trouble with sanctions and blockades is that somebody always tries to violate them. In this case, the Israelis permit humanitarian aid to be sent to Gaza through Israeli ports/checkpoints, but the flotilla was a deliberate effort to bring shipments to Gaza without Israeli inspection. And that left Israel with two choices: abandon the strategy of sanctions and blockade, or enforce them, up to and including the use of force if the blockade met with resistance. This is a corollary of two points I’ve been making for years: one, that if you send soldiers into anything, you have to know how far you are willing to go to accomplish their mission, and two, that all forms of leverage in international conflict resolution are war by other means, and at the end of the day they are backed by credible threats of force, or they are not.

We have long experience with the problems of sanctions policies and the blockades necessary to enforce them. One is the risk that using force to enforce a blockade risks violence and escalation – think of the Japanese bombing Pearl Harbor partly in response to U.S. economic sanctions (an oil embargo), or the U.S. entering World War I partly in response to Germany sinking ships bound for blockaded Britain, or the U.S. and the Soviet Union nearly coming to nuclear war over JFK’s blockade of Cuba in 1962 during the Cuban Missile Crisis. A second is the drain and diplomatic strain created by long-running blockades: think of the cost of keeping U.S. troops in Saudi Arabia to essentially blockade Iraq from 1991 to 2003, and the tensions that helped inflame. A third is the humanitarian cost, as regimes like Saddam’s and Castro’s and Kim Jong Il’s keep their leadership well-fed and housed while passing the costs of sanctions down to the innocent populace. A fourth is the pervasive risk that sanctions regimes cannot be enforced, especially in countries with multiple borders or where powerful countries openly or tacitly decline to cooperate. North Korea or Iran cannot be sanctioned effectively without the sincere cooperation of Russia and China. Iraq conducted significant trade while under sanctions, and succeeded in corrupting the UN Oil-for-Food program to a shocking but predictable extent. And where the purpose of sanctions is to prevent the targeted state from obtaining weapons, especially nuclear materials, chemical/biogical weapons, rockets or other explosives, the blockade must be airtight or it will be a dismal, lethal failure.

None of these things are reasons never to use sanctions. In some essentially non-violent disputes, like international disapproval of South African apartheid or trade disputes, even partly effective sanctions can provide significant leverage. In some cases, like Israel’s blockade of Gaza and the U.S. sanctions against Cuba, the small size and geographic proximity of the blockaded target make sanctions and blockade more effective. And there are often arguments to be made that the evil of sanctions and blockade are lesser than the evils of the alternatives of either inaction or aggressive, perhaps preemptive, military force and occupation.

But know this: if you argue for sanctions as a tool of international diplomacy, you must be willing to own the consequences, up to and including shooting people who try to run the blockade, perhaps in some cases mistakenly, perhaps in other cases where they were only trying to deliver “humanitarian” aid that’s being blockaded. The incident on the flotilla was the face of sanctions. Remember what that face really looks like before you argue again for their use.