As war progresses between Israel and Gaza, the battle is raising the specter of a wider regional conflict in the Middle East. Whether the militant group Hezbollah — which operates in Lebanon, Israel’s northern neighbor — will become involved, opening a second front of the war, is a central question.

Hezbollah and Israel, longtime enemies, have traded shelling since Hamas’s weekend attack on Israel. Though that has prompted fears of a broader war, skirmishes between Hezbollah and Israel have been routine for decades. And some experts say Hezbollah would be reluctant to fully join the latest fray, for fear of losing what gains it has made in Lebanon.