In an Israel facing existential threats that for decades have been acute and at many moments critical, Ariel Sharon became the country’s father. Two decades ago he was dismissed as a politician without a future. But during the past few years he was embraced by the always-anxious citizens of Israel as the parent who could shield them from what they felt were unrelenting, spreading, escalating and cataclysmic threats to their very existence.

Through his decisive unilateralism, Mr. Sharon protected Israel’s existence. And through his bold confidence in himself, he instilled Israelis with a confidence in their future. Small wonder that Israelis across the political spectrum responded to his massive stroke last week with widespread fear and grief. And small wonder that they can’t imagine anyone who could fill his shoes.

How did this happen? Mr. Sharon’s prospects in public life seemed to end when he was found by an Israeli court to have been “indirectly responsible” for the massacre of Palestinians by Lebanese Christian militiamen. And for years he was poundingly vilified by Israel’s left, as well as by much of its center, as the cause of Israel’s problems for having engineered the building of Jewish settlements in the West Bank and Gaza. How did he become the man to whom most Israelis, including most of those on the left, came to look for protection from threats against their lives and the existence of their state? And what does his likely passing from his country’s political scene mean for the future of Israel?

What Mr. Sharon did after he took office as prime minister in 2001 was to assess the situation realistically, abandon his previous ideology and launch a new strategy for national survival.

Toward the Palestinians, he dropped the bilateral strategy that the whole world demanded but that clearly wouldn’t work and adopted a unilateral strategy that the world condemned but that, in his view, was the only one that would save the state.

A bilateral strategy, launched by the Oslo Accords of 1993, was one in which peace was to be achieved, and ultimately a Palestinian state, through a partnership between the Israeli leadership and the Palestinian leadership. As the Israeli leadership would loosen controls and give up territory in the West Bank and Gaza, the Palestinian leadership would stop terrorism and focus on security and nation-building.

The trouble was that there was no Palestinian leadership, at least not one willing or able to pursue a collaboration for peace. There were powerful and popular factions, especially Hamas, that insisted that Israel was illegitimate, swore they would never recognize its right to exist, and were openly committed to destroying the Jewish state through terrorism. And there was the Palestinian Authority, established by the Oslo Accords, that formally recognized Israel but that, often, found it useful to have the terrorists do its dirty work.

Mr. Sharon understood that bilateralism wasn’t working and wouldn’t work against Palestinian terrorism, and that Israel had to protect itself unilaterally. He built a security barrier between Israel and the West Bank and ordered the assassination of terrorist leaders. As a result, fewer and fewer Israelis were blown up.

Israelis, in turn, realized that, whatever Mr. Sharon was doing, even actions they would have opposed before, was saving them. Not all agreed that the barrier should have been put up, or that it should have followed exactly the route that Mr. Sharon used. And not all agreed with every counter-terrorist action he ordered. But the vast majority felt that Mr. Sharon’s strategy was helping them and their country survive.

Israelis also felt that Mr. Sharon was protecting them from another existential threat, a demographic one. And he did this, too, by recognizing reality, abandoning his former ideological positions and acting unilaterally in the interest of preserving his country. He got out of Gaza, the most concentrated Palestinian population center, and probably was preparing to get out of most of the West Bank as well. Just as Israelis didn’t want to rule over millions of Palestinians, they didn’t want to incorporate them into Israel as citizens, at which point those Palestinians, given the fact that Israel is a democracy, would have voted Israel out of existence.

Israelis on the country’s political left had been warning about this dilemma for many years, but couldn’t have done anything about it because they were committed to a bilateral partnership with a Palestinian leadership that couldn’t function or whose strategy was to refrain from functioning. Mr. Sharon went ahead unilaterally; he ignored that leadership and just left Gaza, though it cost him the support of his own party. Most Israelis were now hoping for a similar action in the West Bank, with Mr. Sharon choosing which borders would best protect the country.

With regard to yet another existential threat to Israel, an Iran whose president was insisting that Israel should be “wiped off the map” even as it was rapidly gaining nuclear capability, Mr. Sharon let Israelis believe that, just as he followed unilateral policies that protected them against Palestinian terrorists, he would follow such policies to protect them against destruction by an Iranian nuclear strike. What exactly would he do? Would he – could he – successfully blow up Iranian nuclear installations the way a previous Israeli government, in 1981, blew up the Iraqi nuclear facility at Osirak, ending its operation? No one was sure. But Israelis felt that Mr. Sharon was more ready and able to take decisive action on his own than any other Israeli leader.

It was this kind of boldness, the self-confidence to take steps unilaterally to ensure the security of his people and the survival of their country, that, in turn, inspired the confidence and trust of the vast majority of Israelis and led them to see Mr. Sharon as their protective father. It’s hardly surprising that, shocked by the prospect of losing him, and with no one on the political horizon who seems capable of replacing him, they face their future with a heightened sense of trepidation.

Dr. Reich is the Yitzhak Rabin Memorial Professor of International Affairs, Ethics, and Human Behavior at George Washington University; a senior scholar at the Woodrow Wilson International Center for Scholars, and a lecturer in psychiatry at Yale University. He was the director of the United States Holocaust Memorial Museum from 1995 to 1998.