History is what we make for ourselves. There is no inevitable march of history in any direction. The ideology that believed in the inevitability of historical processes died a discredited death as the Berlin Wall crumbled in 1989.

A united Ireland is not demographically inevitable. The continuance of the Union is not inevitable. Peace is not inevitable. Nor is conflict. Nothing is inevitable in history. For the Good Friday agreement to work, we must decide to replace the politics of aspiration with the politics of accommodation.

The parties to the agreement acknowledge "the substantial difference between our continuing and equally legitimate, political aspirations". They go on to say that their object is "reconciliation and rapprochement".

These two differing aspirations - a united Ireland and unqualified membership of the United Kingdom - are, as the agreement says, equally legitimate, but they are opposite and not reconcilable in terms. If the two communities continue to define themselves in ways that cannot be reconciled with one another, there will be difficulties in achieving the rapprochement sought by the agreement between them.

The agreement contains a voting procedure which requires parties to designate themselves as adherents of one of two opposite aspirations. As long as the two communities define their very existence in irreconcilable terms, arguments on almost any topic which appeal to one community will tend to create fear in the other community.

This is a difficulty in selling the agreement. While it is difficult for unionists not to argue for the agreement on the basis that it has "secured the union", it is equally difficult for nationalists not to argue for it as "a stepping stone" towards a united Ireland.

One Republican supporter of the agreement even went so far as to claim in an Irish-American newspaper that the British government endorsement of the agreement "is a declaration of intent to leave, and that's all we were ever looking for".

These arguments are self-defeating because nationalists and unionists can read one another's newspapers and see one another's spokespersons on television. In this context, "aspirational" arguments which appeal to one community frighten the other. That is why it is so important to create new aspirations to which both communities can assent.

"Peace" is, was and will continue to be, an aspiration to which both communities can assent. The genius of the originators of the peace process was that they made "peace" itself the issue and "peace" was something with which no one could really disagree. Peace provided an emotional cement that bound together loyalists and republicans, unionists and nationalists in a common endeavour.

The difficulty is that any aspiration, once achieved, ceases to provide the same emotional cement it created while still being striven for. If the agreement is to succeed we must create a new common aspiration, binding together unionists and nationalists in their common work. Without the emotional cement of common aspirations, there is a real risk that new institutions will revert to negative factionalism, fed by the constant reminders that the defining aspirations of the two communities are contradictory.

There is a better way. The power of the European ideal - of building a structure of co-dependence which would make war in Europe impossible - sustained the world's greatest co-operative political achievement of the second half of this century - the European Union. People did not cease to be French or German but they created a new common aspiration which transcended their national aspirations.

Northern Ireland, Ireland as a whole and Britain now need a similar ideal. It should be to build a structure of co-dependence between unionists and nationalists in Northern Ireland, within Ireland as a whole, and between Britain and Ireland, so that physical conflict between nationalism and unionism becomes impossible, just as the EU has made physical conflict between France and Germany impossible.

Such a concept is difficult for both nationalists and unionists to accept.

The ideology of Irish nationalism has long been a separatist one. Separation from Britain has been the nationalist ideal. Close co-operation with Britain now, in order to build a structure of peace between nationalism and unionism, goes against the grain of nationalist history.

But North-South bodies go against the grain of unionist history too. Since partition, the unionist ideology has stressed separation from the South as the touchstone of belief. The nature of unionism and nationalism will have to change if the agreement is to lead to a lasting settlement.

The new aspiration - building a structure of co-dependence which makes conflict impossible - must replace the traditional aspirations which made conflict inevitable up to now. The agreement and the institutions it creates must become the focus of a new loyalty. This agreement is not the means to some other end. The agreement must be seen as an end itself. Unless that happens, every ordinary proposal from one side will be seen by the other through a prism of suspicion.

For example, unionist proposals to promote an internal Northern Ireland initiative on drugs, or to build a co-operative venture with Scotland on tourism, would tend to be seen by nationalists, not on their merits, but as a means of deepening the Union.

Equally, a nationalist proposal for all-Ireland or cross-border co-operation on hospitals would be seen by unionists, not on its merits, but as a possible loosening of the Union and accelerating a process towards a united Ireland.

This game of suspicion would gradually corrode the new institutions. That is why we must make the agreement itself the new focus for allegiance on the island of Ireland.

This will not be easy. Even within the agreement, there is a requirement that all members of the assembly, once elected, designate themselves as "nationalist", "unionist" or "other", so as to measure cross-community support in assembly votes. This is necessary but I hope it does not entrench division by defining parties on the basis of ultimate aspirations (which conflict with one another) rather than short-term aspirations (which may coincide with one another).

The rules may aggravate this by making it disadvantageous for a member to designate himself or herself in the "other" category, where his or her vote will only count in initial votes and not in determining minimum levels of communal support.

These difficulties show the need to go beyond procedural devices and build a new and genuine cross-community allegiance which transcends the old categories. This will take time, and may seem impossibly idealistic at the moment, but it is the logic of the peace process.

John Bruton is leader of the Fine Gael party