Explained | What international law says about Israel's invasion of Lebanon

Since the start of Israel's invasion of Lebanon last month, debate has swirled about the wisdom of Israel's two-front strategy amid the ongoing conflict in the Gaza Strip, the threat that the fight against Hezbollah poses to civilians and the risk it could ignite a regional war with Iran.

But perhaps even more fundamental is the question of whether Israel's invasion is legal under international law.

Israel says that it has the right to defend itself, citing a year of rocket attacks by Hezbollah from Lebanese territory. Some of its critics disagree.

"Legality is very much in the eye of the beholder," said Hugh Lovatt, an expert on international law and armed conflict at the European Council on Foreign Relations. "Does Israel's right to self-defense trump Lebanon's right to sovereignty? We can go around and around this circle."

Additionally, some experts say, self-defense has its legal limits, especially if Israel's use of force in Lebanon is disproportionate to the threat it faces or if it fails to prioritize the protection of civilians.

"You have a right to self-defense, but you have to exercise this self-defense in a certain way," said Judge Kai Ambos, a law professor at the University of Göttingen in Germany, who serves on a special tribunal at The Hague that prosecutes war crimes committed in Kosovo during the 1990s. "It's not limitless."

Determining the legality of Israel's invasion can be murky, the experts said, allowing for a wide range of interpretation that, in most cases, would have to be settled by a court or the United Nations Security Council. But it is rare for courts or the Security Council to address these types of questions, and if they did, the process would be unlikely to yield a swift conclusion -- or any at all.

Here is a look at some of the legal questions surrounding Israel's invasion.

What does international law say?

Article 2(4) of the United Nations Charter "prohibits the threat or use of force and calls on all members to respect the sovereignty, territorial integrity and political independence of other states." But Article 51 of the charter also makes clear that member states have a right to defend themselves from armed attacks.

There are more complications. Lebanon is a sovereign state, but Israel says it is fighting against Hezbollah, which is both a militant group and an influential player in Lebanon's government. (Israel and the United States consider it a terrorist organization.)

Hezbollah was founded in the 1980s, with help from Iran, to fight Israel's occupation of Lebanon at the time. Israel and Hezbollah have been in violent conflict ever since, including a bloody war in 2006 that also saw Israel invade Lebanon.

Most recently, after Hamas led the Oct. 7 terrorist attacks on Israel last year, Hezbollah began firing rockets and missiles at Israel in solidarity with Hamas, its ally. Israel returned fire, and the two sides carried out nearly a year of tit-for-tat exchanges. Civilians and combatants in Israel and Lebanon were killed, and between Oct. 7 and the Israeli invasion, more than 150,000 people were displaced on both sides of the border.

Some experts say the invasion is legal because Lebanon allows Hezbollah to use its territory to strike Israel.

In light of the Hezbollah rocket and missile attacks, "Israel has the legal right to take self-defense measures against Hezbollah, and probably also against the Lebanese State," Amichai Cohen and Yuval Shany, two Israeli law professors, wrote in an essay published Friday for the Lieber Institute for Law and Warfare at the United States Military Academy.

In an email to The New York Times, Shany said the United States and its allies had used similar reasoning to "operate in Syria against ISIS and in a number of other countries who had al-Qaida presence."

"As we note in our piece," he added, "the case for self-defense in Lebanon is even stronger."

The UN charter says the right to self-defense is valid only "until the Security Council has taken measures necessary to maintain international peace and security." The Council has tried -- and largely failed -- to secure a cease-fire between Israel and Hezbollah since 2006, the previous time Israel launched a ground invasion in Lebanon.

A Security Council resolution that year prohibited foreign forces from entering Lebanon without the government's consent. Some UN officials have declared the current ground invasion a "violation of Lebanese sovereignty and territorial integrity," and a panel of UN experts said it amounted to "Israel's latest breach of international law."

But that same 2006 resolution also ordered Hezbollah to leave a buffer zone in southern Lebanon where the United Nations deployed peacekeepers to prevent further conflict with Israel. Hezbollah did not leave, and the UN peacekeepers have been unable to stop its frequent rocket attacks on Israel over the past year.

(A UN peacekeeping mission operating along Lebanon's border has come under Israeli fire twice this past week, UN officials have said.)

Humanitarian Legal Protections

Separate from questions about the legality of Israel's invasion, every country has a legal obligation to safeguard civilians during warfare.

Even if Hezbollah places military targets in civilian buildings, for example, experts say Israel must consider the safety of the noncombatants inside when it conducts airstrikes. (International law does not distinguish between ground invasions and airstrikes -- the measure is "use of force," according to Oona A. Hathaway, a professor of international law at Yale University.)

The United Nations says more than 1,500 people have been killed in Lebanon by the Israeli military in the past two weeks, including hundreds of deaths in a single day in September, during one of the most intense air raids in recent warfare.

"While it is difficult to make definitive legal assessments of individual attacks from far away," said Janina Dill, co-director of the Oxford Institute for Ethics, Law and Armed Conflict, in an email, "the use of heavy explosives in densely populated areas of Lebanon and attacks against residential buildings where Hezbollah militants are suspected to hide, which have caused hundreds of casualties, many of them women and children civilians, raise very serious concerns about compliance with these rules."

Nearly 1 million people have been forced to flee their homes in Lebanon, a humanitarian crisis that many fear will soon rival the one in Gaza.

Humanitarian laws of war, including the Geneva Conventions, require military forces to give civilians ample warning to flee before attacking. Israel has issued evacuation alerts for large sections of south Lebanon, though in some cases it has given people as little as two hours to leave their homes before striking.

Israel is also required to consider whether displaced people can be relocated safely. For example, the United Nations says more than 250,000 people have fled from Lebanon to Syria, which is still ravaged from a civil war that began in 2011.

Enforcing the Law

It is difficult, if not impossible, to enforce international laws of war when nations disagree on how, or even whether, to act against violations.

The International Court of Justice allows lawsuits to be brought against states that are accused of violating specific treaties, like the genocide charges that South Africa has lodged over Israel's military operations in Gaza.

Were a case to be referred to the international court over its military operations in Lebanon, Ambos said, Israel could refuse to comply with the outcome. That would potentially send the legal dispute to the Security Council to be enforced.

The UN General Assembly could also be asked to seek a resolution, Hathaway said. But it does not have the authority to take action against Israel except to call on member states to do so.

"The question is, who will enforce this?" Ambos said.

That doesn't mean international law is without value. The laws, he said, impose moral standards to protect civilians that no state would admit to have willingly violated. "Are we better off or worse off with these mechanisms, even if they are not enforced?" Ambos said. "Without these laws, we would be worse. The law is there, and the state must at least justify its actions."