"The objective of this stage is to destroy the terrorist infrastructure of the Hamas in the area of operation," an IDF statement said, "while taking control of some of rocket launching area used by the Hamas, in order to greatly reduce the quantity of rockets fired at Israel and Israeli civilians."
To achieve those goals, Israel attacked the tiny Gaza Strip from four directions on land and, possibly, by sea. Sources told ABC News the plan was to encircle Gaza City, Gaza's biggest population center with 250,000 people, by morning.
Israeli forces aimed essentially to cut the Gaza Strip into four slices, to go after almost 300 Hamas concrete tunnels and defensive installations, and to try to draw Hamas fighters onto the streets where they'd be more vulnerable, ABC News' Simon McGregor-Wood reported.
The Israeli government also authorized the call up of thousands of reservists, signaling that the conflict could widen even more.
Israeli officials said the country's forces did not intend to reoccupy the Gaza Strip, which its forces occupied until 2005.
"We think it's gonna be a long operation," said Maj. Avital Leibovich, an Israeli military spokesman. "This is due to the fact that Hamas, for the past two years, has invested a lot of time and effort in turning Gaza into a barrel of explosives."
Israeli forces have gone into Gaza at least twice before since pulling out in 2005 -- including a fairly large maneuver in 2006 and a smaller one 10 months ago.
Barak noted Israel also was keeping "an open eye on the sensitive situation on our northern border" with Lebanon, where Israel fought a battle against Hezbollah in 2006, vowing to stop the group's missile-launching capabilities. In that case, even after Israeli troops invaded Lebanon, Hezbollah launched an increasing number of missiles at Israel.
This time, the IDF statement suggested Israeli forces were entering the much smaller Gaza Strip with formidable force.
"Large numbers of forces are taking part in this stage of the operation, including infantry, tanks, engineering forces, artillery and intelligence with the support of the Israel Air Force, Israel Navy, Israel Security Agency and other security agencies," the statement said.
"The residents of Gaza are not the target of the operation," the statement added. "Those who use civilians, the elderly, women and children as 'human shields' are responsible for any and all injury to the civilian population. Anyone who hides a terrorist or weapons in his house is considered a terrorist."
Earlier Saturday, at least 10 people were killed in an Israeli air strike on a mosque in northern Gaza, Palestinian medical officials told the Associated Press.
It was not immediately clear whether those killed in the mosque attack in the northern town of Beit Lahiya were militants or civilians.
In a separate attack, the Israeli army claims to have killed Abu Zakaria al-Jamal, the latest senior Hamas leader to have perished since the attacks began eight days ago.
According to the United Nations, Israeli forces have hit 600 targets since the air strikes began.
The beginning of artillery attacks Saturday before the ground invasion raised the possibility of additional civilian casualties caused by the relative inaccuracy of artillery shells compared with precision missile strikes, according to an AP report.