Prof. Samuel Estreicher (NYU) on The Laws of War, as to the Hamas-Israel War

From the article, by a leading American academic who teaches and writes about, among other things, Foreign Relations Law and the law of war:

Those who argue Israel does not have a right of self defense make two claims: First, they argue that Israel's "inherent right" is nullified because it is the "occupying power" of the Gaza Strip; and, second, they argue that the right of self-defense in Article 51 of the UN Charter applies only to force against other states, not non-state actors like Hamas.

Both arguments are unavailing. It is highly doubtful that Israel qualifies as an "occupying" power of the Gaza Strip because once it withdrew all military forces from Gaza in 2005, it has exercised no authority over the territory, which is a requirement under international law before assuming the responsibility of an occupying power. UN groups like the Human Rights Council counter that the status of "occupier" still applies because Israel has imposed stringent limits on travel to and trade with Gaza. And yet Egypt has (mostly) sealed off Gaza's other border without being termed an occupying power.

In any event, even if Israel were an occupying power, that status would not justify Hamas' intentional attacks on civilian targets in Israel or prevent Israel from using force to protect itself. For example, it was not considered illegal for the United States, as the occupying power of Iraq in the immediate aftermath of the 2003 Iraq War, to use force against Iraqis who were attacking U.S. forces within Iraq.

The second argument against Israel's right of self-defense—that it cannot invoke this right against non-state actors like Hamas—is equally untenable.

Article 51 of the UN Charter states that nothing shall "impair the inherent right" of self-defense possessed by all member states. In recent years, many governments, including the United States, have adopted the view that this right of self-defense applies against non-state actors like al-Qaeda and ISIS in situations where the government of the state in which the non-state actors are operating is "unwilling or unable" to prevent attacks by the non-state actor. The case of the Gaza conflict is much stronger for Israel than the "unwilling or unable" rationale; Israel is protecting against Hamas' direct, indiscriminate killing, beheading, torture and hostage-taking of Israeli civilians.

Of course, Israel must exercise its right of self-defense in conformity with international humanitarian law (IHL), also known as the laws of war or armed conflict. This body of customary international law, which was codified in the 1949 Geneva Conventions and their subsequent protocols, limits the use of force to the following situations: military necessity; where a distinction is made between combatants and non-combatants; and where the use of force is proportionate to the concrete military objective sought to be achieved.

Israel Defense Forces' (IDF) attacks on Hamas military targets in Gaza, thus far, almost certainly comply with these principles. However, Hamas' indiscriminate killings, beheadings, maimings, hostage-taking, and rocket attacks against Israeli civilians almost certainly violate these principles. Their violations extend to Gazans as well. Hamas places its military installations and fighters next to and underneath its civilian population centers, in the hope that the Israeli military response will result in collateral civilian deaths and produce battalions of additional supporters for the Palestinian cause. The weaponization of Gazan civilian lives is reprehensible, and we must keep in mind that it is an intentional, indeed central, strategy for Hamas. That strategy, whatever its harm to Israelis, is the direct cause of Gazan civilian deaths, injuries, and destruction of their property….