A classified report by the National Intelligence Council found that even a large-scale assault on Iran launched by the United States would be unlikely to oust the Islamic republic’s entrenched military and clerical establishment, a sobering assessment as the Trump administration raises the specter of an extended military campaign that officials say has “only just begun.”

The findings, confirmed to The Washington Post by three people familiar with the report’s contents, raise doubts about President Donald Trump’s declared plan to “clean out” Iran’s leadership structure and install a ruler of his choosing.

The report, completed about a week before the United States and Israel initiated the war on Feb. 28, outlined succession scenarios stemming from either a narrowly tailored campaign against Iran’s leaders or a broader assault against its leadership and government institutions, the people familiar with its findings said. In both cases, the intelligence concluded that Iran’s clerical and military establishment would respond to the killing of Supreme Leader Ayatollah Ali Khamenei by following protocols designed to preserve continuity of power, these people said.

The prospect of Iran’s fragmented opposition taking control of the country was described as “unlikely,” said the people, who spoke on the condition of anonymity to discuss a classified report.....

Current and former U.S. officials say they see little sign, at least so far, of a mass popular uprising in Iran or of significant fissures within the government or security forces that will result in a new regime. Iran’s security forces killed thousands of protesters during demonstrations in January fueled by the country’s abysmal economy. The guidance from Trump to the Iranian people has been to shelter in place until the U.S.-Israeli bombing campaign concludes.