The Belfast Agreement definitely kills off de Valera's Constitution, at least insofar as it affects the North. The "national territory" is now no longer to be defined as the "whole island" of Ireland. It was precisely this phrase which led to the Supreme Court ruling of March 1990 which founded the "constitutional imperative" towards Irish unity.

If this was not good enough for unionists, there is also the neglected fact that the new Anglo-Irish international agreement which accompanies the agreement includes an explicit recognition of Northern Ireland's status as part of the United Kingdom - the denial of which caused such a furore in the North at the time of the Anglo-Irish Agreement of 1985.

As senior republicans reluctantly admit, the agreement formalises the legitimacy of British rule in the North based on consent. But if it marks a decisive break within nationalism away from the stale mould of de Valera's thought, this poses the question even more sharply: what is living and what is dead in contemporary unionist thought?

In the late 1930s, the ultra-liberal Gen Hugh Montgomery (founder of the Irish Association) entered into a correspondence with John Andrews, a prominent unionist Cabinet member, later to be Prime Minister of Northern Ireland (1940-43) and later still to be Grand Master of the Orange Lodge of Ireland (1948-54).

Despite this Orange association, Andrews - as he makes clear in the correspondence - was well aware of his family's liberal Presbyterian political tradition. Indeed, his brother Thomas, one of the doomed heroes of the Titanic tragedy, was a well-known and widely admired exemplar of the non-sectarian, scientific and progressive culture of Belfast.

But Andrews insisted that he could not rethink his hostile attitude to Irish nationalism - although he favoured in principle harmonious relations between North and South - until Irish nationalists dropped their absolutist claims as embodied in the 1937 constitution.

Gen Montgomery felt that to ask nationalists to do this was to ask them to admit "as a precondition" that on the key issue - the democratic legitimacy of partition - they were wrong. But if the general was right, that is precisely what mainstream Irish nationalism has now done and the implication is clear: the more friendly relationship which Andrews in principle advocated should be delivered in return.

Quite apart from the actual detail of cross-Border co-operation discussed in the Belfast Agreement - and much of it is practical common sense - we need at a minimum in the North a return to that spirit expressed by the staunchly unionist Northern Whig in 1925, when it declared that at partition Northern Ireland had not surrendered its title as part of Ireland, nor had it renounced its share of Irish traditions of art, learning, song, sport and sciences.

But although one would not guess it from the language of the unionist rejectionists, the union is a two-sided relationship between Northern Ireland and the rest of the United Kingdom. If unionism is to flourish after the agreement, it can survive only as part of the culture of a liberal multinational state, the UK.

There should be no difficulty here; the new arrangements for the British/Irish Council ensure that the province should not become a dull backwater as in the "good old days" of Stormont.

As the poet Gerald Dawe has recently put it in The Rest is History (1998): Protestants who believe in the Union, who see, in other words, London as their capital, and Manchester, Birmingham, Leeds, Edinburgh and Glasgow as like-minded and as "British" as themselves, and all those who believe in democratic rights, should have absolutely nothing to lose or fear from speaking their mind on the argument about why the Union should remain.

While acknowledging the "outdated nationalist rhetoric which obscures the acute, deep-seated economic and cultural realities" which continue to link the islands together, Dawe, in this essay, puts his finger on a flaw in unionism when he dismisses a certain type of Ulster unionism as "rant about a mysterious cultural link".

It is this unionism which is now under serious challenge - a unionism which asserts an abstract Britishness but has only one thing to say about or to the British state, that it is driven solely in relation to Ireland by the desire to stop bombs in London, even though the history of the peace process clearly reveals that the British were reluctant converts to the approach based on absorbing the "extremes" which triumphed in the Downing Street Declaration.

This is a unionism which evokes Churchill's "stand against appeasement" but forgets that the real Churchill also did business with Michael Collins and, indeed, only really warmed to Ulster unionism when it produced a leader, in Sir James Craig, who was prepared to do the same.

It would be foolish to deny that the project of selling the Belfast Agreement to unionists is currently facing considerable difficulties. Many feel that they cannot give their consent to the early release of prisoners by voting Yes - though there is as yet no firm indication that this will have a decisive influence on party choices at next month's assembly election.

Above all, many unionists have not come to terms with the project of allowing Sinn Fein a soft landing. The reality of this is quite clear: the mainstream republican movement is now settling for terms which fall short not just of the original objective - British withdrawal and coercion of Ulster Protestants into a united Ireland; the agreement is also a long way short of the joint authority/"Britain to be a persuader" for Irish unity notions which lurked around the Hume/Adams document of 1992.

It is even significantly short of the "minimum requirements" (freestanding North/South bodies and retention of the territorial claim) position articulated earlier this year. Mr Adams is making the best of a bad job with some style, but Sinn Fein is now corralled within institutions which, while they recognise the Irish dimension and equality agendas in a new way, are based above all on the principle of unionist consent.

The agreement is the velvet coffin for the republican project - the reactivation of a more radical republican agenda is only possible if unionists split, fall out among themselves and engage in a futile attempt to wreck institutions sanctioned by a solid majority of the Northern Irish people and, of course, by the British government whose multi-billion pound subvention sustains the economic and social life of the province.

This reality is the forcing principle of a new mood in unionist politics but, at the moment, there are still many unionists who have not got the message: modernise, go through the pain barrier, accept that any conceivable assembly arrangement would give Sinn Fein significant proportional influence and gain the prize of stability, a Northern Ireland at ease with the rest of Ireland and a respected part of the UK.

Paul Bew is professor of Irish politics at Queen's University, Belfast