This week, the Israeli military launched a border operation. Their goal is to destroy a network of attack tunnels they say were built by the Iranian-backed terrorist group Hezbollah.

The action threatens to push the bitter enemies closer to open conflict for the first time since a 2006 war. Israeli military says it has increased forces along the border and warns Hezbollah to keep its distance from the tunnels. “Hezbollah knows well that whoever attacks the state of Israel will pay a very heavy price,” says Prime Minister Benjamin Netanyahu.

Israeli military officials view the northern front as the country’s biggest security concern—with archenemy Iran entrenched in Syria and Hezbollah gaining strength in Lebanon.

Lt. Col. Jonathan Conricus, an Israeli military spokesman, says the army has been tracking the Hezbollah tunnel project since 2014. He says Tuesday’s operation, codenamed “Northern Shield,” has been in the works for a year and a half.

A tunnel revealed Tuesday stretches some 200 yards from a home in the Lebanese village of Kafr Kela across the border and exits in farmland near the Israeli border town of Metula. The tunnel had electricity, a ventilation pipe, and a communications cable. Metula residents told Israeli media they heard digging noises for years and had reported their suspicions to the military.

Under the terms that ended the 2006 war, Hezbollah is not supposed to operate in southern Lebanon. But Israel has long accused the group of violating those rules.

(Serbian United Nations peacekeepers patrol the Lebanese side of the Lebanese-Israeli border in the southern village of Kafr Kela, Lebanon. AP Photo)