Israel’s Supreme Court on Monday struck down a key component of Prime Minister Netanyahu’s contentious judicial overhaul, a decision that threatens to reopen the fissures in Israeli society that preceded the country’s ongoing war against Hamas.

Those divisions were largely put aside while the country focuses on the war in Gaza, which was triggered by a bloody cross-border attack by Hamas terrorists. Monday’s court decision could reignite those tensions, which sparked months of mass protests against the government and had rattled the cohesion of the powerful military.

There was no immediate reaction from Mr. Netanyahu.

In Monday’s decision, the court narrowly voted to overturn a law passed in July that prevents judges from striking down government decisions they deem “unreasonable.” Opponents had argued that Mr. Netanyahu’s efforts to remove the standard of reasonability opens the door to corruption and improper appointments of unqualified cronies to important positions.

The law was the first in a planned overhaul of the Israeli justice system. The overhaul was put on hold after Hamas terrorists carried out their October 7 attack, killing more than 1,200 people and kidnapping 240 others. Israel immediately declared war, and is pressing forward with its offensive.

In an 8-7 decision, the Supreme Court justices struck down the law because of the “severe and unprecedented harm to the core character of the State of Israel as a democratic country.”

The justices also voted 12-3 that they had the authority to overturn so-called “Basic Laws,” major pieces of legislation that serve as a sort of constitution for Israel.

It was a significant blow to Mr. Netanyahu and his allies, who claimed the national legislature, not the high court, should have the final word over the legality of legislation and other key decisions. The justices said the Knesset, or parliament, does not have “omnipotent” power.

Mr. Netanyahu and his allies announced their sweeping overhaul plan shortly after taking office a year ago. It calls for curbing the power of the judges — from limiting the Supreme Court’s ability to review parliamentary decisions to changing the way judges are appointed.

Mr. Netanyahu and his allies said the changes aim to strengthen democracy by limiting the authority of unelected judges and turning over more powers to elected officials. But opponents see the overhaul as a power grab by Mr. Netanyahu, who is on trial for corruption charges, and an assault on a key watchdog.

Before the war, hundreds of thousands of Israelis took to the streets in weekly protests against the government. Among the demonstrators were military reservists, including fighter pilots and members of other elite units, who said they would stop reporting for duty if the overhaul was passed. The reservists make up the backbone of the military.

While the reservists quickly returned to duty after the October 7 attacks in a show of unity, it remains unclear what will happen if the overhaul efforts are revived. A resumption of the protests could undermine national unity and affect the military’s readiness if soldiers refuse to report for duty.

Under the Israeli system, the prime minister governs through a majority coalition in parliament — in effect giving him control over the executive and legislative branches of government.

As a result, the Supreme Court plays a critical oversight role. Critics say that by seeking to weaken the judiciary, Mr. Netanyahu and his allies are trying to erode the country’s checks and balances and consolidate power over the third, independent branch of government.

Mr. Netanyahu’s allies include an array of ultranationalist and religious parties with a list of grievances against the court.

His allies have called for increased settlement construction in Judea and Samaria, annexation of the territory, perpetuating military draft exemptions for ultra-Orthodox men, and limiting the rights of LGBTQ+ people and Palestinians.

The Biden administration previously urged Mr. Netanyahu to put the plans on hold and seek a broad consensus across the political spectrum.

The court issued its decision because its outgoing president, Esther Hayut, is retiring and Monday was her last day on the job.