Israel’s defense failures may change strategy toward Hamas and Gaza.

Hamas’s attack on Saturday took Israeli intelligence officials by surprise, particularly the methods the militants used to enter and leave Israel, according to a senior defense official familiar with the information collected about the group.

The broad attack, mostly successful from Hamas’s point of view, revealed some significant failures by the Israeli defense establishment. It also may change Israel’s overall strategic approach to Hamas and the Gaza Strip, said the official, who asked not to be identified when discussing security matters.

And that could have a far-reaching effect on the entire Middle East.

Until now, Israel has contained Hamas and Gaza with a strategy that hinged on an intelligence network that would warn against Hamas’s moves, and on the power of the Israeli Army to repel a ground invasion by Hamas. In the Hamas attack on Saturday, these two safeguards failed.

Israel is traditionally perceived as the strongest intelligence power in the region, with extensive coverage of the Gaza Strip. And in recent months, Israeli intelligence did repeatedly warn that a military conflict could flare up because Iran and affiliated militias have perceived Israel as weakened by the nation’s profound divisions over the judicial overhaul being pursued by the ultraright governing coalition of Prime Minister Benjamin Netanyahu, according to four senior defense officials.

Still, while Israeli intelligence collected some indications that Hamas was planning a major operation, they were far from forming a clear picture, one of the officials added.

Israel, the official said, did not pick up on the elaborate preparations that were most likely needed for the 250 Hamas militants tasked to lead the assault, and target military bases, cities and kibbutzim.

American officials, too, said that both Israel and the United States had known a Hamas attack at some point was possible, or even likely. But they said there was no specific tactical warning of the strikes on Saturday, no sign that would have allowed Israel to take specific measures.

Many questioned why Israel and the United States were blindsided. Mick Mulroy, a former C.I.A. officer and senior Pentagon official, said the complexity of the attack by Hamas indicated that it would have required much preparation.

“There were likely indications of the buildup of munitions and the preparation of the assault force, and there was cyber activity in Israel prior to the assault,” Mr. Mulroy said.

Since the Israeli withdrawal from the Gaza Strip in 2005, Hamas has transformed from a militant organization to the leader of a territory with many characteristics of a state. The group has started rounds of fighting with Israel every few years, which usually have not lasted more than a week. These attacks include firing rockets on Israeli cities and trying to kidnap or kill Israelis. But nothing has been as extensive as the Saturday attack.

For its part, Israel in past years has responded with its enormous firepower, usually from aircraft, against targets in Gaza and has tried to assassinate the organization’s senior officials. But it has launched very limited ground maneuvers.

The Israeli strategy has been to contain the fighting against militants in the Gaza Strip, as long as Israel’s fatalities were not too high, which might oblige it to engage in an all-out ground invasion.

Four successive Israeli prime ministers decided that the price of invading and occupying the Gaza Strip to crush Hamas rule would be too high, in the lives of Israeli soldiers and Palestinians, and that the toll of governing millions of residents there would be too costly.

Israel continued to act this way even though it knew that both Hamas and Palestinian Islamic Jihad have received funding, training, weapons and advanced combat and intelligence gear from Iran, three officials said, and that the militant groups were becoming stronger.

The surprise attack on Israel came almost 50 years to the day to the start of the Yom Kippur War, which began with a surprise attack by Syrian tank columns and Egyptian brigades. That made it even more surprising that Israel was not more on guard.

The defense official said this was most likely not a coincidence but a careful choice by Hamas to pick a date perceived as a national trauma. The intelligence surprise, as well as Hamas’s ability to cross the border and cause heavy losses, is strikingly reminiscent of the 1973 war.

Israel has invested enormous resources in getting intelligence about Hamas, gathering significant information about most of its initiatives and targeting many of its leaders.

But Saturday was not the first time that Hamas has managed to surprise Israeli intelligence. In June 2006 when a Hamas squad entered Israel, attacked a group of soldiers, killed two and kidnapped the soldier Gilad Shalit, Israeli intelligence did not know about the attack, or where Shalit was being held for more than five years. Israel eventually paid the highest price it had ever paid to secure a P.O.W.

That deal brought intense controversy within Israel, which could flare again with reports that dozens of Israeli soldiers and civilians had been captured.

The Israeli Defense Forces, even though they were aware of the possibility of a ground invasion by Hamas to seize military bases and civilians along the border, were slow to reach the scenes of violence. Many residents were forced to defend themselves.

The videos Hamas took during the operation and which were immediately distributed on social media presented the Israeli defense establishment as weak, surprised and humiliated.

Israel is now likely to respond with force, and possibly with a ground invasion of Gaza, in the belief that Hamas did not leave it any choice, a senior defense official said.

One key question, which will determine how the crisis unfolds, is whether Hezbollah, the Lebanese militant group, stays on the sidelines or if it activates its fighters to attack Israel. If Hezbollah becomes directly involved the fighting it is likely to become some of the most intense in the region in years.

Julian E. Barnes contributed reporting from Washington.