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If the conflict between Israel and Hamas is an international armed conflict (IAC), there is no question that Israel has the right to blockade Gaza. (
Which is not to say that the manner in which Israel is blockading Gaza is legal. That’s a different question.) The 1909 Declaration Concerning the Laws of Naval War (the London Declaration), the first international instrument to acknowledge the legality of blockades, specifically recognized the right of belligerents to blockade their enemy during time of war. Article 97 of the San Remo Manual does likewise. And there is certainly no shortage of state practice supporting the legitimacy of blockades during IAC (the US blockade of Cuba, for example).
But what justifies a blockade in non-international armed conflict (NIAC)? The London Declaration does not justify such a blockade, because it only applies to “war”– war being understood at the time as armed conflict between two states. Does the San Remo Manual justify it? The Manual is not a picture of clarity concerning when its rules apply, but it does not seem to contemplate non-international sea conflicts. Article 1 speaks of “the parties to an armed conflict at sea,” which does not seem to include NIAC, unless perhaps a rebel group has a navy. (Do any?) Article 2 parallels the Martens Clause in the 1907 Hague Convention (IV) Respecting the Laws and Customs of War on Land, which only applies to IAC. Article 3 acknowledges the right of self-defense under Article 51 of the UN Charter, but — as Marko Milanovic has pointed out — that right is an exception to the prohibition on the use of force in Article 2(4), which only operates between states. And numerous articles in the Manual refer specifically to “belligerent States” (see, for example, 10, 20, 34).
There also appears to be little, if any, state practice to support the idea that a blockade is legally permissible in NIAC. According to the Jerusalem Post, the Israeli government is defending the blockade by citing Yoram Dinstein’s statement that “there are several instances of contemporary (post-UN Charter of the Law of the Seas) practices of blockades, e.g. in the Vietnam and in the Gulf War.” But those were all blockades in IAC. I can’t think of any blockades in NIAC other than Israel’s blockade of Gaza — though readers should feel free, of course, to correct me.
The seeming absence of support for blockades in NIAC is obviously important, because it is difficult to argue that Israel is involved in an IAC with Hamas. First, it is obviously not in a traditional IAC, because Gaza is not a state. Second, not even Israel claims that the conflict has been internationalized by the involvement of another state.
And third, although the Israeli Supreme Court held — controversially — in the Targeted Killings case that armed conflict between an occupying power and a rebel group is international, Israel’s official position is that it not currently occupying Gaza.
Israel’s defense of the blockade thus appears to create a serious dilemma for it. Insofar as Israel insists that it is not currently occupying Gaza, it cannot plausibly claim that it is involved in an IAC with Hamas. And if it is not currently involved in an IAC with Hamas, it is difficult to see how it can legally justify the blockade of Gaza. Its blockade of Gaza, therefore, seems to depend on its willingness to concede that it is occupying Gaza and is thus in an IAC with Hamas. But Israel does not want to do that, because it would then be bound by the very restrictive rules of belligerent occupation in the Fourth Geneva Convention.
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http://opiniojuris.org/2010/06/02/wh...of-gaza-legal/