Israel's blockade of Gaza is illegal irrespective of the manner in which it is imposed because a blockade is an act of war and an occupying power cannot declare war upon the territory it occupies. To do so would conflate the right to initiate war (jus ad bellum) with the laws of occupation (jus in bello) and render useless the distinction of the permissible use of force in each case. This analysis is different in kind from the one that characterizes the blockade as illegal for its contravention of Article 33 of the Fourth Geneva Convention prohibiting collective punishment.
The prohibition on collective punishment stipulates that if indeed Israel is imposing the blockade in order to repress and prevent Hamas mortar and rocket fire than it should do so in a way that does not severely harm the civilians to whom it owes a duty to protect as an Occupying Power. Several humanitarian and human rights law organizations, (e.g., ICRC Gisha, OPT Special Rapporteur) have established the blockade's illegality for its contravention of Article 33 at considerable length. Significantly, this duty is not unique to occupying powers as non-occupying belligerents also have the duty to ensure the welfare of the civilian population. Accordingly, Israel has a duty to protect the civilians in Gaza irrespective of whether or not it remains an Occupying Power. However, specifically because Israel remains an Occupying Power, its blockade is illegal even if Israel were to ensure the welfare of Gaza's 1.5 million inhabitants. Israel's status as an Occupying Power prevents it from invoking legal self-defense, and from using force beyond that permissible during police operations, against Gaza -- the territory it occupies.
As it stands, the existing legal order prohibits an occupying power from initiating force against its occupied territory because where there exists a belligerent occupation, presumably, an armed attack has already occurred in response to which a belligerent initiated force. Therefore Article 51 self-defense is not available to Israel because: "the time when self-defense could be invoked has passed: the resort to force has already occurred, and the situation is now governed by the different regime of international humanitarian law."
That regime is jus in bello, and in particular, the laws of occupation. The laws of occupation place the responsibility for maintaining law and order, and for breaches of said order in Occupied Territories, upon the Occupying Power. The permissible use of force available to an Occupying Power to do so is derived from Article 43 of the Hague Regulations and is significantly more restricted than the force available to belligerents during hostilities. Article 43 limits permissible force to law enforcement or policing purposes. According to Marco Sassoli,
Police operations are subject to many more restrictions than hostilities. To mention but one example, force may be used against civilians only as a last resort after non-violent means have proved unsuccessful in maintaining law and order. As for the use of firearms it is an extreme measure in police operations, while it is normal against combatants in hostilities.
Accordingly, Israel has the right and the duty to police the Gaza Strip but it can neither use force permitted during hostilities, nor can it invoke Article 51 self-defense. As a blockade amounts to an act of war under international customary law, its imposition on Gaza both breaches the limitation on Israel's permissible use of force as well as flagrantly challenges the definition of Article 51 self-defense. Israel dismisses these restrictions arguing that on the one hand, it is no longer an occupying power in Gaza, and on the other, that even if it was, it would still have the right to legal self-defense.
Upon its unilateral disengagement from Gaza in 2005, Israel declared its occupation over and its responsibility for Gaza's civilian population, expired. However according to in international law, occupation hinges on the something called "effective control" which is derived from Article 42 of the 1907 Hague Regulations. The "effective control" test does not require the military presence of the Occupier throughout the territory but rather "the extent to which the Occupying Power, through its military presence, is exerting effective control over the territory and limiting the right of self-determination of the occupied population." The controlling element is whether a belligerent has established its authority and has the ability to exercise it.
Consider that in its Disengagement Plan, Israel reserved the right to use force against Palestinians living in Gaza in the name of preventive and reactive self-defense. Since 2005, Israel has conducted several military operations in the Strip in the name of such self-defense. Consider also that Israel has maintained control of its air space, its seaports, its telecommunications network, its electromagnetic sphere, its tax revenue distribution, and its population registry. Finally, Israel has complete control of Palestinian movement as it controls its five border crossings with Gaza and therefore the ingress and egress of all its goods and people. The confluence of its ongoing control, its continuous military operations, as well as its capacity to redeploy its troops within a reasonable time, demonstrate that Israel remains in effective control of the Gaza Strip. There exists general international consensus affirming the Gaza's ongoing status as an occupied territory and Israel's status as an occupying power. Accordingly, the laws of occupation remain in force.
Israel argues that even if it is still an occupying power that it can invoke Article 51 self-defense and to complete its circle, Israel frames its argument within the framework of the U.S.'s War on Terror. In direct response to Al-Qaeda's attacks on the United States on September 11, 2001 the UN Security Council passed Resolutions 1368 and 1373. The Resolutions affirm that terrorist acts amount to threats to international peace and security and therefore trigger the "inherent right of individual or collective self-defense as recognized by the Charter of the United Nations." Israel has deliberately worked to first cast all acts of Palestinian violence as terrorist acts; secondly to frame those acts as amounting to armed attacks; and thirdly to argue that such armed attack triggers Article 51 self-defense pursuant to Resolutions 1368 and 1373 irrespective of the West Bank and Gaza's status as Occupied Territories.
The International Court of Justice dealt with this challenge in its Advisory Opinion on the Legal Consequences on the Construction of a Wall in the Occupied Palestinian Territory. There, the Court reasoned that Article 51 contemplates an armed attack of one State and against another State and "Israel does not claim that the attacks against it are imputable to a foreign state." Moreover, the Court held that because the threat to Israel "originates within, and not outside" the Occupied West Bank, "the situation is thus different from that contemplated by Security Council resolutions 1368 (2001) and 1373 (2001), and consequently, the Court concludes that Article 51 of the Charter has no relevance in this case." While there is considerable controversy about whether an armed attack must be imputed to a foreign state, beyond the challenge posed by Israel and its political agents, there is no controversy that where the laws of occupation apply, Article 51 self-defense cannot be imposed.
To assert otherwise is arguably unfair as it affords the occupying power both the right to use police force in a Territory and, if and when it feels that those powers are inadequate, it expands its use of force by invoking a broader right to self-defense. Moreover, an occupying power should not be able to justify its use of military force as self-defense in response to a breakdown in order within a Territory for which it is responsible for maintaining order. In doing so an occupying power would be conflating, and rendering useless, two otherwise distinct legal regimes of jus in bello and jus ad bellum. Nonetheless, Israel continues to challenge the legal order and to insist that international law is not mature enough to handle its security concerns. To this end, Israel's challenge and insistence is a deliberate attempt to change the law.
*This is based on a paper entitled, "Is it Wrong or Illegal? Israel's Blockade of Gaza in International Law" to be published by the Issam Fares Institute, American University of Beirut.