Right before October 7th, Israel was in the midst of a huge political upheaval because of the looming judicial overhaul. Gantz was the leader of the opposition, and the main political rival of Netanyahu. The country was extremely divided, almost verging on civil war.
Right after October 7th, Israelis were focused on unity, survival, and combatting Hamas, Hezbollah, and other threats.
Gantz joined the war cabinet mostly to signal that whatever disagreements he and his party have with Netanyahu [of which there are many], he still prioritizes national unity in the face of existential threat.
This unity was fragile, and by now the political infighting in Israel is back in full swing.
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