Israel does not have to respond.
And no Iran did not start this any more than Iraq had weapons of mass destruction.
But Israel likely does, and I don’t remember the last inspection reports.
Iran Helped Plot Attack on Israel Over Several Weeks
The Islamic Revolutionary Guard Corps gave the final go-ahead last Monday in Beirut
https://www.wsj.com/world/middle-east/iran-israel-hamas-strike-planning-bbe07b25
Iranian general whose assassination triggered weekend attack on Israel helped plan Oct. 7: report
Hundreds of Hamas fighters trained in Iran ahead of Oct 7 attack on Israel: Report
Iran: Oct. 7 attack was revenge for killing of Soleimani in 2020
Iran began collaborating with Hezbollah and Hamas on Israel attack plan ‘over a year ago’: report