Donald Macintyre: A historic withdrawal - but there are many reasons to be cautious about the future

The nightmare haunting Gaza moderates is that Hamas will claim the credit for driving Israel out

The momentousness of the step, or of the fact that it has been ordered by Ariel Sharon, the quintessential architect of the settlements, is not in doubt. This is the first significant abandoning of Jewish settlements built so relentlessly in Palestinian-inhabited territory occupied since 1967. It creates a precedent of immeasurable importance. The word "historic" is for once perfectly appropriate to describe what will start happening in Gaza next week.

Several reasons to be cautious remain about assuming that disengagement will create its own irresistible dynamic for further and rapid change, for example disengagement in the West Bank. For a start, even among Mr Sharon's supporters there are different explanations of his goals. Some ministers explain the reversal in Gaza in purely demographic terms; that the old warrior finally realised what those towards the left of Zionism had been telling him for a long time, that the long-term preservation of a democratic Jewish state could hardly be guaranteed if its writ ran over all the territory it occupied in 1967for the simple reason that Arabs would be a majority in such a state.

This argument is underlined by figures published yesterday showing that the population of Jews in territory under Israel's control is already just below 50 per cent. What the same survey shows, however, is that merely by losing Gaza, the proportion will jump to 58 per cent, guaranteeing a Jewish majority for another 20 years.

So if demography were all Mr Sharon was worried about, he could envisage with equanimity the West Bank remaining under Israeli control for long beyond his political lifetime. Which as, it happens, was just what his canny lieutenant Dov Weisglass appeared to be suggesting when he said in a famous interview last year - while Yasser Arafat was still alive - not only that 180,000 West Bank settlers should thank the Prime Minister for getting out of Gaza because they could remain in place but that disengagement would "freeze" the political process.

Against that Ehud Olmert, Mr Sharon's deputy, in an expansive session with foreign journalists this week, insisted repeatedly that Gaza was not intended as "a trade-off with the West Bank". Instead he reinforced what Mr Sharon has himself said publicly, that it offers the Palestinians a real opportunity for a return to the long moribund road map to peace if Mr Abbas fulfils Israel's requirement of him disabling the armed factions and eliminating attacks on Israel. In other words that the step reflected real change rather than, as Mr Weisglass had appeared to imply, continuity.

But even if Mr Olmert is right, there are serious obstacles. The subtext of both Israel and the international community is that by making a "success" of Gaza, Mr Abbas can show the world that the Palestinians are ready for the full state to which Mr Sharon says he is committed. But this is a tall order. The substance of the belated "co-ordination" talks between Israel and the Palestinians is highly detailed - from putative scanners to replace the cripplingly slow security checks on cargo leaving Gaza for Israel, through the future of settler greenhouses to the location of customs points between Egypt and Gaza.

But the talks are really about the [Israeli] security/[Palestinian] freedom trade-off. The painfully slow progress of the talks - not helped by internal disagreements in both camps - has a direct effect on Mr Abbas's chances of making Gaza "work". The PA can hardly escape the blame for the chaotic state of its security services, any more than it can for corruption and the absence of a robust judicial system. But it would certainly help Mr Abbas - including in his efforts to restrain the armed factions - if Gaza had the access to outside markets its economy desperately needs. The nightmare haunting moderate Palestinians in Gaza is that Hamas will strengthen its popularity by claiming it drove Israel out, while Mr Abbas will have nothing to show for what Israel continues to insist is a "unilateral" step.

But Mr Abbas will also badly need a political horizon, some evidence that a return to the negotiating table, or at least of further progress in the West Bank, is in sight. And here the medium-term prospects are also doubtful. Benjamin Netanyahu's stunningly self-serving resignation this week has revived talk of Mr Sharon breaking with Likud and forming a party of the "centre". But the best bet is that Mr Sharon will seek to beat off Mr Netanyahu's certain challenge for the Likud leadership, and that to do that he will tack back to the right to win back the party's anti-disengagement majority, making the chances of substantive further progress all the less, at the very least ahead of the next Israeli elections, and maybe well beyond that.

Which is where the last, and perhaps most crucial, unknowable, comes in: the staying power - and perception of its own interests - of the US. There are strong signs that Condoleezza Rice has been pressing Israel to do more to help Mr Abbas in the co-ordination talks. But whether she and President Bush are prepared to go all out to ensure that disengagement creates a political momentum is so far unclear. Especially since they largely share Israel's insistence on Mr Abbas first performing the daunting task of clamping down once and for all on the armed factions and are trying, in the person of General William Ward, to equip him to do it.

Some European diplomats have begun canvassing the possibility of a fresh UN Security Council resolution after disengagement, backed by the US and doing what Washington has so far failed to do, by making clear the minimum requirements of both sides in any final-status settlement. One of many suggestions is that it could conform to the very broad principles in the People's Voice plan agreed between Ami Ayalon, the former head of Shin Bet and Sari Nusseibi, the principal of Al Quds University, under which any territorial changes in the pre-1967 borders would be matched by a one-for-one land swap.

If UN resolutions could solve this conflict, of course, they would have done so generations ago. But it would be a definite advance for the US; and it might provide some form of political horizon, however misty. Of course, if such a resolution were also underpinned by a clear recommendation to both parties to put such a peace plan to a referendum of both Israelis and Palestinians, there is every chance that it would secure a majority among both peoples. It is part of the nature of this conflict that this doesn't make it any likelier to happen.