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).
UPDATE: As a number of commenters have pointed out, Lincoln’s blockade of the Confederate States of America (CSA) during the Civil War is a relevant historical precedent. But I think that the Civil War blockade actually supports the argument I’ve made above. As noted in the Lincoln section of the University of Virginia’s Miller Center of Public Affairs, the international community viewed the blockade as an act of war that required the CSA to be formally recognized as a belligerent, thus effectively transforming what was previously a NIAC into an IAC:
The first crisis occurred when England issued a proclamation of neutrality, which rested upon the logic of the Union’s declared blockade. According to English reasoning, although Lincoln proclaimed the rebels to be insurrectionists and thus not recognizable under international law as a belligerent power engaged in war, his declared blockade was an act of war, which would have to be conducted against a sovereign state. Thus Lincoln had actually granted belligerency status to the Confederacy and thereby forced foreign powers to do the same. By proclaiming neutrality, England afforded the Confederacy the status of a belligerent power. Other European nations followed England’s lead. Belligerency status gave the Confederacy the right, according to international law… to contract loans and to purchase arms from neutral nations. It also allowed England to provide safe harbors for both Union and Confederate warships and merchant vessels, to build blockade runners and warships for the Confederacy, and to formally debate in Parliament the merits of active intervention.
L.C. Green, one of the great IHL scholars, agrees with this analysis. If this is still the state of the law — and I don’t know whether it is — it would be possible to argue that Israel’s conflict with Hamas is an IAC and Israel is thus entitled to blockade Gaza.
But there’s a catch — and a big one. If the “cost” of the blockade is formally recognizing Hamas as a belligerent, maintaining the blockade would mean recognizing Hamas fighters as privileged combatants. (Just as the armed forces of any state are privileged combatants.) That would be fundamentally unacceptable to Israel, because Hamas fighters would then be entitled to attack Israeli combatants and would have to be treated as POWs upon capture.