Made up largely of veterans and reservists, the group that calls itself Brothers in Arms started nine months ago as a protest movement against the government of Prime Minister Benjamin Netanyahu. It helped to organize giant weekly demonstrations against his plans to give the government unprecedented control over the judiciary. And many of its members vowed that they would not serve in the army if called.

But everything changed after Hamas brutally attacked Israel on Oct. 7. Not only did the reservists show up for duty, but Brothers in Arms has also transformed into the largest nongovernmental aid agency in Israel to help those affected by the war.

It is helping the military secure protective vests and helmets. It is feeding and clothing some of the perhaps 60,000 Israelis displaced from the area near Gaza after the Hamas assault. It is providing therapy to some and feeding the livestock and picking the produce of others, as well as opening a center to take care of abandoned dogs.

It has worked with A.I. experts and volunteers to comb videos and social media accounts to try to verify whom Hamas took as hostages and work out who survived and who did not. It has also provided any intelligence it can gather to the Israeli military.

But the more the group is seen positively by the Israeli public, the more those who support Mr. Netanyahu and his policies see it as a growing political force, even a threat.