Ask former Israel prime minister Ehud Olmert whether the 2005 plan he drove to withdraw Israeli troops and settlers from the Gaza Strip was an attempt to further the creation of a Palestinian state alongside Israel, or to avoid one, and the response is a kind of verbal shock and awe.

Unilateral disengagement was Olmert’s brainchild, developed when he was part of Ariel Sharon’s government in the early 2000s, and implemented just before Sharon’s incapacitating stroke put Olmert in charge. The decision involved pulling all Israeli troops and Jewish settlers out of Gaza, with the aim of ending the occupation that had begun with Israel’s crushing victory over Arab neighbours in the Six Day War of 1967.

The idea was controversial from the start. Some argued it was naive, especially after Gazans promptly elected Hamas, a party dedicated to Israel’s destruction. Binyamin Netanyahu, a cabinet minister at the time and now prime minister, resigned in protest. He warned disengagement would lead to more attacks on Israel and the loss of its 1967 territorial gains. Others saw it as a cynical ruse for Israel to avoid having to negotiate a political settlement.

“There is only one person that can give you the answer, only one, me,” Olmert said in an hourlong interview at his Tel Aviv office. “My vision was clear-cut, explicit, without any doubts: a two-state solution.”' He believes it was Netanyahu’s decision to build up Hamas and weaken its West Bank rival, the Palestinian Authority, to eliminate any credible partner to continue settlement negotiations that changed the whole direction of travel.

There is reason to believe Olmert because, in 2008, three years after the disengagement, he arguably came as close as Israel and the Palestinians ever have to reaching a two-state settlement. Had they succeeded, history might have been quite different.

Who was right then matters now, because the answer will set the course for how to proceed after Israel’s military has finished a seemingly inevitable ground invasion of Gaza to crush Hamas, in the wake of its horrific October 17 terrorist attack on Israel. For 15 years, Olmert says, Netanyahu allowed Hamas to arm while giving Israel’s religious and nationalist Right free rein to increase settlements in the occupied West Bank, leaving nonviolent Palestinians with no hope and no “horizon for peace”.

It is that policy of creeping annexation and deliberate Palestinian humiliation that must now be reversed. Olmert says the goal of disengagement back in 2005 was to take a unilateral step towards the deoccupation of first Gaza and then the West Bank, because it seemed peace negotiations were stalemated and something was needed to break the logjam. It is a topic on which he seems genuinely passionate. He says his dominant motivation for disengagement was “the sour fruits of occupation, the sour fruits in terms of what we do to them, and the sour fruits in terms of what it does to us, to the Israeli psyche, to Israeli attitudes, to a sense of decency, of humanity, which we are losing.”'

Olmert tells the story of a handwritten letter he once received as prime minister from a cabinet member in the Palestinian Authority who had gone to Jerusalem to meet with an Israeli counterpart. He took his young son with him, to see the Al-Aqsa mosque. They were stopped at a checkpoint, where the father was asked to submit to a strip search, which, as Olmert pointed out, “means totally naked, and they are checking all the holes in your body”. The Palestinian official, deeply humiliated, turned around and went home to write his letter.

Olmert says he called in person to apologise for the insult, most importantly to the official’s son, “because when I talked to this guy on the telephone, I thought that the son, who witnessed what they were doing to his father, he will become a Hamas-nik!” Olmert said: “How many hundreds of thousands of Palestinians have been through the same experiences that have created this hatred and hostility.”

A second reason behind disengagement, after the ill-effects of occupation, was demography, Olmert says. It was becoming clear that a growing number of Palestinians thought they might be better served by a single state, in which they demanded a vote, than by a two-state solution that gave them control of only part of the Holy Land. Because of their faster population growth, Palestinians already by this time would have commanded a majority in a one-person-one-vote system across the whole of Israeli-controlled territory. That, said Olmert, meant either the end of democracy in Israel, as a Jewish minority suppressed the vote of a Palestinian majority, or the end of a Jewish state. Disengagement from Gaza took the wind out of those sails.

In Olmert’s view, a two-state solution remains the only viable way out of the dilemma that has confronted Palestinians and Israeli Jews since at least 1967, and the question is how to get back on track for it. That cannot be done, Olmert says until Hamas — an equivalent to Isis or al-Qaeda — has been erased from Gaza. “You cannot negotiate with them,” he said. “You have to kill them. It is either they, OK? Or you.”

After that, Israel needs to reverse its policies and create a horizon for Palestinians, roll back the West Bank’s Jewish settler movement, accept that each side will have to make territorial concessions and build the Palestinian Authority back up as a credible partner, says Olmert. Now, as in 2005, he is aware this will not be easy. A joint Palestinian/Israeli poll in January found support for a two-state solution down to about a third of the population on both sides, with two thirds of Palestinians and 53 per cent of Israeli Jews now opposed to the idea.

But that is a challenge for a strong politician, not a verdict. What’s clear is that Netanyahu cannot lead such an effort. His Palestinian policy collapsed in horrific style on October 7. It is unlikely to be Olmert, either. He resigned from office in 2008 to fight corruption allegations, for which he was eventually convicted and spent time in jail. But he is right that the only way to get out of a cycle of war is to get back on the path towards a two-state solution.

What Olmert cannot answer is why, if the humiliation of a father in front of his son risks creating a Hamas recruit, the killing of thousands of Palestinians as collateral damage in a war to destroy Hamas will not do the same. I suspect we both know there is no answer to the question.

• Marc Champion is a Bloomberg Opinion columnist covering Europe, Russia and the Middle East. He was previously Istanbul bureau chief for the Wall Street Journal