“This is something that is going to make it very difficult for our diplomatic presence there, our military presence there,” Mr. Kim said in an interview.

General Suleimani, who led the Islamic Revolutionary Guards Corps’ Quds Force, a special forces unit responsible for Iranian operations outside Iran’s borders, was long a figure of intense interest.

He was not only in charge of Iranian intelligence gathering and covert military operations, he was regarded as one of Iran’s most cunning and autonomous military figures. He was also believed to be very close to Ayatollah Khamenei, and was seen as a potential future leader of Iran.

The United States and Iran have long been involved in a shadow war in battlegrounds across the Middle East — including in Iraq, Yemen and Syria. The tactics have generally involved using proxies to carry out the fighting, providing a buffer from a direct confrontation between Washington and Tehran that could draw America into yet other ground conflict with no discernible endgame.

The potential for a regional conflagration was a basis of the Obama administration’s push for a 2015 agreement that froze Iran’s nuclear program in return for sanctions relief.

Mr. Trump withdrew from the deal in 2018, saying that Mr. Obama’s agreement had emboldened Iran, giving it economic breathing room to plow hundreds of millions of dollars into a campaign of violence around the region. Mr. Trump responded with a campaign of “maximum pressure” that began with punishing new economic sanctions, which began a new era of brinkmanship and uncertainty, with neither side knowing just how far the other was willing to escalate violence and risk a wider war. In recent days, it has spilled into the military arena.

General Suleimani once described himself to a senior Iraqi intelligence official as the “sole authority for Iranian actions in Iraq,” the official later told American officials in Baghdad.