This is the 5th war Gazans have started, however the massive firing of rockets and the murder of 1200 Israelis on Oct 7 marks a massive change and therefore a notable starting point. Rod Gillis is correct that Sinwar along with Deif initiated the attack.
One could go back to the founding of Hamas (1988) or the Muslim Brotherhood (1928), which Hamas is a branch of. (The relationship is noted in the preamble of the genocidal Hamas covenant.)