Ismail Haniyeh was one of the most senior leaders of Hamas for the past two decades, in recent years running the militant group’s political operations from exile in Qatar.

On Tuesday, Mr. Haniyeh was in Iran with other senior members of Iran’s “axis of resistance” — which includes Hamas in Gaza, Hezbollah in Lebanon and the Houthis in Yemen — to attend the inauguration of Iran’s newly elected president.

As Hamas’s political leader, he was central to the group’s high-stakes negotiations and diplomacy, including the stalled cease-fire deal negotiations with Israel.

Here is what we know:

Leader of Hamas in Gaza

Mr. Haniyeh was named the leader of Hamas in Gaza in 2006. That year, he briefly served as prime minister of a Palestinian unity government, which was dissolved after months of tension that included armed conflict between Palestinian factions.

In 2017, he was named the leader of Hamas’s political bureau at a time when it was trying to soften its public image as it jockeyed for influence among Palestinians and internationally.

Mr. Haniyeh led Hamas from Qatar and Turkey in recent years. He was among the negotiators in ongoing talks between Israel and Hamas, mediated by Egypt, Qatar and the United States, to end the war in Gaza in exchange for hostages captured in the Hamas-led attack on Israel.