Multiple legal assessments of the Gaza flotilla raid were published subsequent to the event. International law experts (and non-lawyers) differed over the legality of the action by Israel. The force necessary to respond to violent resistance and whether the force that was used was proportionate were disputed.

Approximately one year after the event, the UN investigative committee for the 2010 Flotilla to Gaza concludedReport of the Secretary-General's Panel of Inquiry on the 31 May 2010 Gaza Flotilla Incident that (1) the blockade was legal, based on the principle of self-defense, (2) Israel was "justified in stopping vessels even outside its territorial waters," (3) Israel's decision to board the vessels with such force was "excessive," (4) Israeli forces "faced significant, organized and violent resistance from a group of passengers" that required them to use force for their own protection, and (5) the loss of nine lives was "unacceptable." It recommended that Israel immediately report its use of force to the United Nations Security Council for resolution as specified in Chapter VII of the United Nations Charter.

Some, such as law experts Harvard Law School Professor Alan Dershowitz, Chicago Law School Professor Eric Posner, and Johns Hopkins International Law and Diplomacy Professor Ruth Wedgwood, said that the naval blockade, the boarding in international waters, and the use of force were in accord with long-standing international law. Dershowitz compared the blockade with the U.S. blockade of Cuba during the Cuban Missile Crisis and Posner with the Coalition blockade of Iraq during the first Gulf War.

The United Nations High Commissioner for Human Rights, the International Committee of the Red Cross, the National Lawyers Guild's International Committee, Istanbul Bilgi University's Law Department Dean Turgut Tarhanlı and other experts have concluded that the blockade was itself illegal, or agreed with University of Dundee international law professor Robin Churchill that the boarding on the high seas was illegal even if the blockade were lawful, or agreed with international law professor Said Mahmoudi that the use of force was disproportionate and the raid was therefore illegal even if the blockade and the boarding in international waters were lawful. An investigation by a panel of legal experts convened by the UN determined that the use of force by the Israeli military was disproportionate, that the Israeli military violated international law, and found clear evidence sufficient for war crimes prosecutions under the Fourth Geneva Convention - which defines humanitarian protections for civilians in a war zone and prohibits total war.

The issue of possible violation of international law was discussed at the UN Security Council. The United States blocked a draft resolution criticizing Israel proposed by Turkey, the Palestinians, and Arab nations.

Legality of blockade

Arguments opposing legality of blockade The United Nations Charter requires that nations avoid the use of force when settling external conflicts, and that human rights be respected with regards to internal conflicts. Nations which use force in self-defense are required to immediately report these measures to the United Nations Security Council so that it can fulfill its role of ensuring peace.

The United Nations High Commissioner for Human Rights, Navanethem Pillay, has condemned Israel's blockade of Gaza on multiple occasions, calling it "a direct contravention of international human rights and humanitarian law. It must end now”.

The "United Nations Fact Finding Mission on the Gaza Conflict" in its Goldstone Report said:

1733. The Mission ... considers that the series of acts that deprive Palestinians in the Gaza Strip of their means of subsistence, employment, housing and water, that deny their freedom of movement and their right to leave and enter their own country, that limit their rights to access a court of law and an effective remedy, could lead a competent court to find that the crime of persecution, a crime against humanity, has been committed.OHCHR: Report of the United Nations Fact Finding Mission on the Gaza Conflict (Paragraph 1733)

The International Committee of the Red Cross said that Israel's blockade violated the Geneva Conventions and stated that it constituted a collective punishment imposed in clear violation of Israel's obligations under international humanitarian law.OHCHR: Gaza closure: not another year! ICRC 14-06-2010 News release 10/103

Dr. Turgut Tarhanlı, dean of the Istanbul Bilgi University's Law Department, cited the concept of innocent passage, under which vessels are granted safe passage through territorial waters in a manner which is not "prejudicial to the peace, good order or the security" of the state. He said that:

the Convention on the Law of the Sea stipulates that a coastal state may consider intervention if a ship is engaged in arms and drug smuggling, the slave trade or terrorist activities. However, the case with the aid boats is totally different. They set sail in accordance with the Customs Act and are known to be carrying humanitarian aid, not weapons or ammunition. According to the Convention on the Law of the Sea, Israel was not entitled to launch a military operation against the boats and activists.

With regard to the Gaza–Jericho Agreement, Diana Buttu, a Palestinian-Canadian lawyer and former spokesperson for the Palestine Liberation Organization, said that Israel declared the Oslo Accords dead in 2001, and actually breached the agreements, so that a call to the applicability of the Gaza-Jericho Agreement is not plausible.

Douglas Guilfoyle, a lecturer at University College of London since 2007 who authored Shipping Interdiction and the Law of the Sea in 2009, told Aljazeera that, while a blockade is a recognized tool of warfare, a close look should be taken as to whether there was proper notification of the Gaza blockade, and whether it inflicted "excessive damage to the civilian population in relation to the concrete military advantage expected".

Amnesty International, in a report dated June 1, 2010, included the opinion that "As a form of collective punishment, Israel's continuing blockade of Gaza is a flagrant violation of international law."

The European Parliament, in a June 14, 2010 resolution, called the Gaza blockade "collective punishment in contravention of international humanitarian law" and demanded its immediate cessation.

Arguments supporting legality of blockade

Alan Dershowitz, professor of Law at Harvard Law School, wrote that the legality of blockades as a response to acts of war “is not subject to serious doubt.” He likened Israel's maritime blockade of Gaza to U.S. naval actions in Cuba during the Cuban Missile Crisis, which the U.S. had deemed lawful though not part of an armed conflict.

Similarly, Allen Weiner, former U.S. State Department attorney and legal counselor at the American Embassy in The Hague, and now a Stanford Law School professor, said "the Israeli blockade itself against Gaza itself is not illegal".

Ruth Wedgwood, a professor of International Law and Diplomacy at the School of Advanced International Studies at Johns Hopkins University, said that under the law of armed conflict, which would be in effect given Hamas's rocket attacks on Israel and Israel's responses, Israel has "a right to prevent even neutrals from shipping arms to [Hamas]".

Eric Posner, international law professor at the University of Chicago Law School, noting that the raid had "led to wild accusations of illegality", wrote that blockades are lawful during times of armed conflict (such as the Coalition blockade of Iraq during the first Gulf War), and that "war-like conditions certainly exist between Israel and Hamas". He compared Israel's blockade to the Union blockade by the Union against the Confederacy (a non-state) during the U.S. Civil War. The U.S. Supreme Court later affirmed the legitimacy of that blockade.

Philip Roche, a partner in the shipping disputes and risk management team with the London-headquartered international law firm Norton Rose, also said: "On the basis that Hamas is the ruling entity of Gaza, and Israel is in the midst of an armed struggle against that ruling entity, the blockade is legal." The basis for that is the law of blockade, derived from international law that was codified in the 1909 London Declaration concerning the Laws of Naval War, and which was then updated in 1994 in the San Remo Manual on International Law Applicable to Armed Conflicts at Sea--"a legally recognized document". He addressed the charge by Human Rights Watch that the blockade of a terrorist organization constitutes a collective penalty against civilians, ostensibly violating Article 33 of the fourth Geneva Convention, by saying "This argument won't stand up. Blockades and other forms of economic sanctions are permitted in international law, which necessarily means that civilians will suffer through no fault of their own."

International law Professor Ed Morgan of the University of Toronto, likewise, noting that it is clear that Israel and Hamas are in a state of armed conflict, which has been noted by the General Assembly to the Human Rights Council in its Goldstone Report, wrote that a blockade of an enemy's coast is an established military tactic. He pointed out that it is recognized as a means at the Security Council’s disposal under Article 42 of the UN Charter, and is similarly set forth in Article 539 of the Canadian Forces manual Counter-Insurgency Operations. He wrote: Having announced its blockade, Israel had no obligation to take the ships’ crew at their word as to the nature of the cargo. The blockading party has the right to fashion the arrangements, including search at a nearby port, under which passage of humanitarian goods is permitted.

U.S. Vice President Joe Biden said "Israel has a right to know – they're at war with Hamas – has a right to know whether or not arms are being smuggled in. It's legitimate for Israel to say, 'I don't know what's on that ship. These guys are dropping ... 3,000 rockets on my people.'"

Abbas Al Lawati, a Dubai-based Gulf News journalist on board the flotilla, opined that Israel is likely to cite the Gaza–Jericho Agreement (Annex I, Article XI) which vests Israel with the responsibility for security along the coastline and the Sea of Gaza. The agreement stipulates that Israel may take any measures necessary against vessels suspected of being used for terrorist activities or for smuggling arms, ammunition, drugs, goods, or for any other illegal activity.

Professor Wedgwood opined that the goal of the flotilla was to: "denude Israel of what it thinks it was guaranteed in the 1993 Oslo Accords which preceded the Gaza-Jericho Agreement, which is the control of the external borders of Gaza and West Bank.... The problem ... is that you could easily have a rearming of Hamas, which caused a terrible conflict."

Legality of enforcing blockade on the high seas

Arguments opposing legality of high seas enforcement Robin Churchill, international law professor at the University of Dundee in Scotland, said there was no legal basis for boarding the ships as they were in international waters. A group of Israeli lawyers petitioned the Supreme Court of Israel charging that Israel had violated the United Nations Convention on the Law of the Sea by capturing the boats in international waters, but a Supreme Court ruling signed by Justice Dorit Beinish rejected those suits outright.Aviad Glickman,High Court rejects flotilla suits: Soldiers defended their lives, Ynet News, June 3, 2010