Israel originally said its missile strikes on Gaza were a response to rockets fired by Hamas into Israel as tensions grew over the killing of three teenagers from a West Bank settlement and the revenge murder of a Palestinian boy.
The aerial bombardment escalated to a ground incursion after Hamas rejected a ceasefire proposed by Egypt.
But Israel now says its prime military target is a series of tunnels built by Hamas to store weapons and to infiltrate its fighters for attacks in Israel.
Last Thursday, Israel says it foiled an attack by 13 fighters using a tunnel that stretched 800ft beyond the separation fence in the south of the territory. It then ordered in ground troops.
How extensive is this tunnel network?
The Israeli Defence Forces says it has found 13 tunnels so far, of which it has destroyed half, but that there are many more. Some are more than a mile long and are fitted with electricity and lined with concrete supporting walls.
Four have been used by Hamas for attacks on Israel since Thursday, resulting in the death of six Israeli troops.
Is this the first we have known of it?
No. In the past the tunnels into Israel, which could only be used realistically for hit-and-run attacks, were overshadowed by the strategically more important tunnels under the southern border into Egypt, used to smuggle in weapons, people and goods. Egypt's army has now destroyed more than 1,000 of these tunnels.
Does this mean Israel won't stop until it has destroyed all the tunnels? Tzipi Livni, the Israeli justice minister and chief negotiator with the Palestinians, said yesterday that there would be no ceasefire until the "tunnels project" had been finished.