Through nearly 10 months of intense war with Hamas in Gaza, Israel has fought a parallel, slower-paced conflict with Hamas’s allies across the Middle East in which all sides have risked major escalation but ultimately avoided dragging the region into a bigger, multi-front war.

The attacks on two of Israel’s leading foes on Tuesday and Wednesday have created one of the biggest challenges to that equilibrium since the fighting began in October.

Israel’s Tuesday night strike on Fuad Shukr, a senior Hezbollah commander in Beirut, was the first time during this war that Israel has targeted such an influential Hezbollah leader in Lebanon’s capital. Hours later, the killing in Iran of Hamas’s political leader, Ismail Haniyeh, was considered the most brazen breach of Iran’s defenses since October.

Taken together, the seniority of the targets, the sensitive location of the strikes and their near simultaneity were viewed as a particularly provocative escalation that has left the region fearing an even bigger response from Iran and its regional proxies, including Hezbollah, the Houthis in Yemen and militias in Iraq. The scale of that reaction could determine whether the low-level regional battle between Israel and the Iranian alliance tips into a full-scale, all-out conflict.

Some analysts said the killing of Mr. Haniyeh, Hamas’s top negotiator, also made a cease-fire deal in Gaza less likely in the immediate future. Israelis hoped that the killing of such an influential leader would eventually help break Hamas’s resolve, making the group more willing to compromise in the long term. But others said that the organization was unlikely to be seriously affected by Mr. Haniyeh’s death.

Despite his title as Hamas’s political leader, Mr. Haniyeh is replaceable, said Joost Hiltermann, the Middle East and North Africa program director for the International Crisis Group.