The sudden death of President Ebrahim Raisi in a helicopter crash comes at a particularly precarious moment for Iran and the Middle East.

Many observers had believed Raisi, 63, was a front-runner to replace Iran’s 85-year-old supreme leader, Ayatollah Ali Khamenei. His death could upend the battle to helm the theocratic regime just as its shadow war with Israel threatens to spill out into an open regional conflict.