The street revolt in Egypt has thrown the Israeli government and military into turmoil, with top officials closeted in round-the-clock strategy sessions aimed at rethinking their most significant regional relationship.

Israel’s military planning relies on peace with Egypt; nearly half the natural gas it uses is imported from Egypt; and the principle of trading conquered land for diplomatic ties began with its 1979 peace treaty with Egypt. Prime Minister Benjamin Netanyahu has met with President Hosni Mubarak of Egypt more than with any other foreign leader, except President Obama. If Mr. Mubarak were driven from power, the effect on Israel could be profound.

“For the United States, Egypt is the keystone of its Middle East policy,” a senior official said. “For Israel, it’s the whole arch.”

The official spoke on the condition of anonymity because Mr. Netanyahu has ordered his ministers and their officials to stay publicly silent on Egypt while events there play out.

Many analysts here said that even if Mr. Mubarak were forced to leave office, those who replaced him could maintain Egypt’s peace treaty with Israel, since it is the basis for more than $1 billion in annual aid to Cairo from Washington and much foreign investment.

But others noted that the best-organized political force in Egypt is the Muslim Brotherhood, which is hostile to Israel and close to Hamas, the Palestinian rulers in Gaza whose weapons-smuggling the Egyptian government works to block.

As the government evacuated the families of envoys from Egypt over the weekend, public affairs broadcasts and newspapers in Israel focused on the unfolding events there. Most of the predictions were dire. Two of three newspapers with the largest circulations, Yediot Aharonot and Maariv, had identical front-page headlines: “A New Middle East.”

It was an ironic reference to the phrase used frequently in the 1990s by President Shimon Peres and other advocates of coexistence who argued that if Israel made peace with its neighbors, a more prosperous and enlightened region would bloom. Events of the past five years — the takeover of Gaza by Hamas, the rise of Hezbollah in Lebanon, Iran’s influence in Iraq and the shift by Turkey toward Iran and Syria — have turned many Israelis rightward, fearing that the more time passes the more the region is against them.

Israelis worry that Jordan is in a precarious state and a successful overthrow in Egypt could spread there. And if the Muslim Brotherhood were to gain power in Egypt, that would probably mean not only a stronger Islamist force in Gaza but also in the West Bank, currently run by the Fatah-dominated Palestinian Authority, as well as in Jordan, meaning Israel would feel surrounded in a way it has not in decades.

If Egypt also turned unfriendly, that would quite likely stop in its tracks any further Israeli talk of peace negotiations with the Palestinians, officials and analysts said. A peace treaty with the West Bank would involve yielding territory and military control to a relatively weak Palestinian Authority. Trading land for peace with autocrats like Mr. Mubarak, some analysts say, is not a sound basis for enduring treaties.

There has long been concern that popular sentiment in Egypt is anti-Israel. Eli Shaked, a former Israeli ambassador to Cairo, wrote in the Yediot Aharonot newspaper, “The only people in Egypt who are committed to peace are the people in Mubarak’s inner circle, and if the next president is not one of them, we are going to be in trouble.”

Mr. Mubarak has just named Omar Suleiman, his right-hand man and the country’s intelligence chief, as his vice president; Israelis would be reassured if he were to inherit power. Other establishment figures, while less friendly to Israel, would most likely maintain some kind of continuity. But Israelis feared that nothing was certain.

They noted that if Mr. Mubarak were to go, Mr. Netanyahu could be left without an ally in the region. Prime Minister Recep Tayyip Erdogan of Turkey has been highly critical of Israel since the Gaza war two years ago and even more so after Israeli commandos killed nine Turks aboard a flotilla trying to break Israel’s blockade of Gaza last May. King Abdullah II of Jordan, while honoring his country’s peaceful relations with Israel, has been critical of Mr. Netanyahu since he took office two years ago and has declined to meet with him as well.

For the military here, a serious change in Egypt means a strategic shift in planning. Giora Eiland, a former national security adviser and a senior fellow at the Institute for National Security Studies at Tel Aviv University, said even if Egypt did not cancel its peace treaty with Israel tomorrow or in five years, a government dominated by the Muslim Brotherhood would mean “you can’t exclude the possibility of a war with Egypt.”

“During the last 30 years,” he said, “when we had any military confrontation, whether in the first or second Lebanon wars, the intifadas, in all those events we could be confident that Egypt would not try to intervene militarily.”

Dan Schueftan, director of the National Security Studies Center at the University of Haifa, said thanks to its treaty with Egypt, Israel had reduced its defense expenditure from 23 percent of its gross national product in the 1970s to 9 percent today and made serious cuts in its army. The relationship with Egypt also allowed Israel to withdraw from Gaza in 2005, since Egypt covered Gaza from the south.

Despite Mr. Mubarak’s supportive relations with Israel, many Israelis on both the left and right are sympathetic to the Egyptians’ desire to rid themselves of his autocracy and build a democracy. But they fear what will follow if things move too quickly.

“We know this has to do with the desire for freedom, prosperity and opportunity, and we support people who don’t want to live under tyranny, but who will take advantage of what is happening in its wake?” a top official said. “The prevailing sense here is that you need a certain stability followed by reform. Snap elections are likely to bring a very different outcome.”

Israeli analysts also noted that Egypt had worked hard to oppose Iranian ambitions, and the loss of Egypt as a counterweight would have consequences.

Mr. Schueftan of the University of Haifa made this point, saying, “If this cornerstone is removed or even in doubt, the overall picture for Israel changes and the threats become much more realistic than before.”