The Israeli military said the focus of its ground operations in Gaza is to destroy the labyrinth of tunnels from the strip into Israel that can be used for launching attacks.

Israeli officials, who revealed that they had been planning for a year to destroy the tunnels, began their ground assault on Thursday, after 10 days of bombardment, when 13 Hamas gunmen emerged from one underground passage near a kibbutz in southern Israel.

Q. What is the strategic value of the tunnels?

A. In March, a former Hamas prime minister, Ismail Haniya, said in a speech before thousands of supporters in Gaza City that tunnels open “a new strategy in confronting the occupation and in the conflict with the enemy from underground and from above the ground.” In June, five Palestinian militants were killed in an explosion in a tunnel in which they had been working. Hamas has long stored weapons underground.