Israel warned that no Hamas leader was safe, but the assassination of the militant group’s political chief, Ismail Haniyeh, in Iran’s capital has sent shockwaves across a region hardened by war and conflict.

Not only did the strike — which Iran and Hamas have blamed on Israel — deepen fears of an all-out war in the Middle East, it has dimmed hopes for a cease-fire deal that could help wind down the catastrophic war in the Gaza Strip and ensure the release of hostages still being…