Non-state armed groups
Hezbollah is both a Lebanese political party with parliamentary representation and an armed organisation with capabilities approaching those of a small state's military. Its arsenal includes precision-guided missiles, drones, anti-ship capability, and a deep network of fortified positions in southern Lebanon. The movement's relationship with Iran is the closest of any non-state actor and is the principal vector by which Iran can credibly threaten northern Israel.
Sustained Israeli ground operation into south Lebanon
Treated as casus belli; would trigger the largest single escalation vector.