An appendix to the letter cited U.S. laws and Administration policies that prohibited the transferring of weapons likely to be used in war crimes, and Spoljaric urged Biden to withhold weapons “that should not be used in densely populated areas, in light of their devastating humanitarian impacts.” She warned Biden that failing to “uphold minimum standards of humanity” would be a “moral defeat” that “future generations are unlikely to forgive us for failing to avert.”

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Two years after Spoljaric’s letter, President Donald J. Trump ordered the U.S. military to join Israel in attacking Iran. Some former senior officials in the Biden Administration told me that, in certain ways, the war in Iran was an extension of the Gaza conflict. Iran had been central to Biden’s thinking about the Middle East, several top advisers said. He’d feared that any public pressure the U.S. put on Israel—any “daylight” between the allies—could invite attacks by Iran or its Lebanese client, Hezbollah. The U.S. would then be compelled to defend Israel in a regional conflagration. Brett McGurk, Biden’s White House coördinator for the Middle East and probably his most influential adviser, championed this position, arguing that rock-solid support for Israel would help contain Iran, whose proxies killed three U.S. soldiers in the region during the first months of the Gaza conflict.

At the same time, the Biden Administration’s staunch public backing of Israel during the Gaza war had created what Philip Gordon, the national-security adviser to Vice-President Kamala Harris, described to me as a “moral hazard”—it had invited the Israeli Prime Minister, Benjamin Netanyahu, to try to expand the battle into a regional one. Even though Biden and his advisers privately told Netanyahu many times not to provoke Iran, the Israelis repeatedly blindsided the Americans by doing exactly that: a January, 2024, strike on Damascus killed five senior Iranian military advisers; an April, 2024, strike on an Iranian diplomatic mission in Syria killed more than a dozen; a July, 2024, strike in Tehran assassinated a top Hamas political leader; a September, 2024, operation in Lebanon detonated explosives planted in thousands of walkie-talkies and pagers carried by Hezbollah fighters; a subsequent air strike killed Hezbollah’s leader, Hassan Nasrallah.