What is Hizbullah?

ISRAEL IS AMASSING troops at its border with the Gaza Strip ahead of an expected ground invasion. At the same time it is evacuating villages near its border with Lebanon for fear that a second front could open in the north. Hizbullah, an Iran-backed Shia militant group based in Lebanon, has exchanged fire with Israel. Troops on both sides, a journalist and at least two Lebanese civilians have been killed. Iran warns that a “pre-emptive action” against Israel by proxies could be imminent. Hizbullah has gone to war before in support of Hamas, the militant group that runs Gaza. Its involvement in the current conflict would mark a significant escalation. What is Hizbullah, and how great is the threat?