As the United States and Israel prepared to go to war with Iran, the head of Mossad, Israel’s foreign intelligence service, went to Prime Minister Benjamin Netanyahu with a plan. Within days of the war’s beginning, said David Barnea, the Mossad chief, his service would likely be able to galvanize the Iranian opposition — igniting riots and other acts of rebellion that could even lead to the collapse of Iran’s government. Mr. Barnea also presented the proposal to senior Trump administration officials during a visit to Washington in mid-January. Mr. Netanyahu adopted the plan. Despite doubts about its viability among...