WASHINGTON — Israel expects its top ally, the United States, to announce as soon as Monday that it’s blocking military aid to an Israeli army unit over gross human rights abuses in the Israeli-occupied West Bank before the war in Gaza began six months ago.

The move would mark the first time in the decades-long partnership between the two countries that a U.S. administration has invoked a landmark 27-year-old congressional act known as the Leahy law against an Israeli military unit.

It comes as the U.S.-Israeli relationship is under growing strain over civilian deaths and suffering in Israel’s war against Hamas in Gaza.

Former Vermont Sen. Patrick Leahy championed legislation that became the Leahy law in the 1990s, saying the U.S. needed a tool to block American military aid and training to foreign security units guilty of extrajudicial killings, rapes, torture and other flagrant human rights abuses.

One of the first targets of the 1997 law was typical of the kind of renegade units that Congress had in mind: a Colombian army unit accused of knowingly killing thousands of civilians in part to get bonuses that were then being offered for killing militants.

Other U.S. laws are supposed to deal with other circumstances in which abuses would obligate blocking military support. Those include a February 2023 order by President Joe Biden dictating that “no arms transfer will be authorized” when the U.S. finds that more likely than not a foreign power would use them to commit serious violations of the laws of war or human rights or other crimes, including “serious acts of violence against children.”

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How the Leahy Law Works

The law requires an automatic cutoff of aid to a military unit if the State Department finds credible evidence that it has committed gross abuses. A second Leahy law says the same for Defense Department training of foreign militaries.

Rights groups long have accused U.S. administrations, including Biden’s, of shirking rigorous investigations of allegations of Israeli military killings and other abuses against Palestinians to avoid invoking such laws aimed at conditioning military aid to lawful behavior by foreign forces.

Israel says its security forces investigate abuses and its courts hold offenders accountable.

Regularly when it comes to U.S. security assistance to countries in the former Soviet Union and in Central and South America and Africa. Not often when it comes to strategically vital U.S. allies.

In 2022, for instance, the U.S. found sufficient evidence of abuses to trigger the Leahy law for police and other forces in Azerbaijan, Kyrgyzstan, Mexico and the Caribbean nation of Saint Lucia.

The administration also has the option of notifying Congress of Leahy law incidents in classified settings to avoid embarrassing key partners.

Administration veterans vouch that no U.S. government has previously invoked it against Israel, says Sarah Elaine Harrison, a former Defense Department attorney who worked on Leahy law issues and now is a senior analyst with the International Crisis Group.

Harrison points to a 2021 treaty in which Israel stipulated it wouldn’t share U.S. military aid with any unit that the U.S. had deemed credibly guilty of gross human rights abuses.

U.S. law points to one way out for an offender: A secretary of state can waive the Leahy law if he or she determines the government involved is taking effective steps to bring the offenders in the targeted unit to justice.

The U.S. still sends billions of dollars of funding and arms to Israel, including a new $26 billion package to support Israel’s defense and and provide relief for the growing humanitarian catastrophe in Gaza. The Senate is expected to pass that this week and Biden says he will sign.