Scoop: White House has discussed U.S. military response if Hezbollah attacks Israel

The White House has been discussing the possibility of using military force if Hezbollah joins the war in Gaza and attacks Israel with its huge arsenal of rockets, three U.S. officials and one Israeli official with knowledge of the situation tell Axios.

Why it matters: Lebanon-based Hezbollah joining the war would dramatically escalate the Middle East's worst conflict in decades — raising the likelihood of mass civilian casualties in Israel and Lebanon and possibly drawing in the U.S.

• The Israeli military — now focused mostly on Gaza — would have a significantly more difficult time fighting on two fronts simultaneously while missiles rain down on its bases and Israel's population centers.

State of play: Since the Gaza war began Oct. 7, tensions between Israel and Hezbollah have been increasing, with daily skirmishes between the parties along the Israel-Lebanon border.

• Iran-backed Hezbollah has fired rockets and anti-tank missiles at Israeli outposts and forces on the border in recent days, while Israeli airstrikes have hit Hezbollah positions. Several Hezbollah operatives and several IDF soldiers have been killed.
• At the same time, both sides have managed to keep the exchange of fire on a relatively low level, avoiding all-out fighting.
• Hezbollah so far has refrained from sending its operatives for a ground assault or from firing its long-range, accurate missiles at Israeli targets.

Driving the news: Since the Gaza war began, the Biden administration has sent public and private messages to Hezbollah and Iran through third parties, warning them not to intervene in the war.

• One of the U.S. officials said the response the Biden administration received from Iran and Hezbollah through intermediaries was that while they don't want an escalation, they might have to intervene if Israel continues its military operation in Gaza.
• The Biden administration followed these messages by sending two aircraft carriers and several other war ships to the eastern Mediterranean, and put more forces on alert for a possible deployment to the region.
• Biden's trip to Israel this week is designed partly to show U.S. support for Israel — and reflect the administration's concern about Hezbollah opening a second front in the Gaza war.

Two U.S. officials said Secretary of State Antony Blinken told Arab leaders in the region, with whom he'd met in recent days, that the U.S. "is not fooling around" by sending so many military assets to the region in support of Israel.

• The scenario of using U.S. military force if Hezbollah were to join the war has come up in several White House meetings in recent days, the officials said. They added that any decision to use force would be made according to the scope of a Hezbollah attack and Israel's ability to respond.
• The two U.S. officials stressed the administration is doing all it can to keep Hezbollah out of the war — but also is preparing for the opposite scenario.
• The White House declined to comment.

Behind the scenes: In a phone call with Israeli Prime Minister Benjamin Netanyahu on Saturday, President Biden expressed concern about the war expanding along the Israeli-Lebanese border, U.S. and Israeli officials said.

• The officials said Biden asked Netanyahu to keep the Israel Defense Forces' (IDF) operations in Lebanon as restraint as possible and warned of a possible miscalculation that could lead to a serious escalation with Hezbollah.

What they're saying: "President Biden — with his public messages, with his private messages to Iran and Hezbollah and with the practical steps he took — made it clear to our enemies that if they think of joining the attack against Israel there will be American involvement and Israel will not be on its own," Israeli national security adviser Tzachi Hanegbi said at a news conference Tuesday.

Between the lines: One of the legal justifications for the U.S. using force in case of a Hezbollah attack would be to protect tens of thousands of American citizens who live in Israel, said Jonathan Lord of the Center for a New American Security (CNAS).

• "Under commonly held understandings of Article 2 of the U.S. Constitution, the president can enter U.S. forces into hostilities to protect Americans abroad," Lord said.
• He added that if Biden were to make that decision, he'd need to notify Congress within 48 hours under the War Powers Resolution of 1973. That would give Biden 60 days to act before Congress' approval to use military force would be required.