Hamas may claim victory after a cease-fire with Israel that was announced on Wednesday, but 15 months of war have left the militant group severely weakened.

Israel’s bombardment and invasion of Gaza, which began weeks after the deadly Oct. 7, 2023, attacks by Hamas, have decimated the 21 battalions that formed the main unit of the group’s military wing, the Qassam Brigades, according to military analysts.

The total number of Hamas fighters killed is unclear. In September, a spokesman for the Israeli military put the number at more than 17,000, including a number of senior commanders.

Hamas’s ability to mount sustained and coordinated operations has also been greatly reduced, the analysts say. Israel has destroyed large quantities of Hamas’s military hardware and disabled many of the tunnels it used to transport and store material.

Israel also seized a strip of land that runs along Gaza’s southern border in May, depriving Hamas of a conduit to funnel military supplies and personnel into the enclave from Egypt.

Hamas has lost many of its senior leaders. Israeli forces killed the commander of Hamas’s military wing, Muhammad Deif, in July; another top leader, Ismail Haniyeh, later that month; and the group’s leader, Yahya Sinwar, in October. Mr. Sinwar had emerged from two decades in Israeli prisons to help plot the Oct. 7 attack.

“Israel worked on the ground for 15 months with a massive force and managed to destroy much of what Hamas had there,” said Ahron Bregman, a former Israeli military officer who is now a political scientist and expert in Middle East security issues at King’s College London.

But he added that Hamas might still have reason to congratulate itself. “Israel has turned Gaza into dust but Hamas is still standing and they have not raised the white flag,” he said.

The Iran-backed regional proxy network that has helped sustain Hamas has also been decimated.

Israel stepped up its assault on the Lebanese militia Hezbollah starting in September, relentlessly targeting the group’s personnel and infrastructure through airstrikes and killing its leader Hassan Nasrallah in September. That month, hundreds of pagers belonging to Hezbollah members exploded, killing or maiming many of the group’s operatives — as well as civilians.

Days later, Israel launched an invasion of southern Lebanon.The invasion ended with a cease-fire in November under which Hezbollah agreed to stop its missile and drone attacks on Israel, which it had started in solidarity with Hamas.

Last month, rebels toppled Syria’s longtime leader Bashar al-Assad, who had been a key element in Iran’s so-called axis of resistance, eliminating Tehran’s main conduit to funnel arms to Hezbollah.