US stands firm on limiting new Nato members to three 's ever open door policy leaves states at odds on membership

Few have any compunction about describing the Nato summit that opens today in Madrid as "historic". A senior aide to President Bill Clinton described it last week as "a watershed event in the post-war history of Europe".

Barring unforeseen hitches, the summit will end with invitations to three aspiring allies - the Czech Republic, Hungary and Poland - to join what is acknowledged to be "the most successful military alliance ever". The structure of the alliance will also be tweaked in recognition of European demands for more autonomy within Nato: European Nato members want to mount their own joint military operations with American know-how, but without American manpower.

All that, within bounds, has been agreed. Despite this, however, and some very hard work by negotiators in the past 10 days, the summit will not be the harmonious celebration of the West's Cold War victory, nor the uncontested rubber-stamping of new members and new methods that some had hoped for.

The US President, who arrived in Madrid yesterday, finds himself in the unaccustomed role of mediator. He will be mediating, moreover, between two groups who barely recognise the other's existence: Europe's out-and- out advocates of Nato expansion, and an increasingly vocal section in the US that opposes the whole idea of Nato expansion. Trying to steer between these two extremes, the US administration has decided that only the Czech Republic, Hungary and Poland should be invited to join in the first wave.

A majority of European allies - notably France and Spain, but not Britain - believe at least five countries should be admitted, including Slovenia and Romania. When Mr Clinton came out in favour of restricting admission to three even before the question had been discussed at Madrid, they accused the US of behaving as though Nato was its preserve.

Washington backtracked a little on the detail, but not on the substance. Officials insisted nothing would be finally decided until Madrid, where they expected "full and frank" discussions. They stressed their support for Nato's "ever open door", reiterating that failure this time did not mean rejection for good, and offered some friendly diplomatic gestures towards the Baltic states. However, US officials also said Washington's stance remained firm and the signs from Paris, which had spearheaded the short-lived revolt, were that France was already in retreat.

The dispute and the impression created in Europe of "hegemonistic" US behaviour, however, were clearly a cause for concern to Washington as the summit approached. Five senior officials were put up for questioning by the media to explain the US position.

Led by Sandy Berger, the National Security Adviser, they argued that the essentials for Nato membership were "irreversible" democratic and market reforms, political control of the armed forces, progress in relations with neighbouring countries, and the will and capacity not just to accept the benefits of Nato membership, but also its military obligations. In other words, they would have to help pay to upgrade their armed forces.

Mr Berger and the others commended Romania for its progress over the past year, but said evidence of the "irreversibility" of its reforms and of political control over the military was not complete. Arguments against Slovene membership were more difficult to marshal.

Officially, there is concern about the capacity of Slovenia's armed forces to fulfil Nato obligations; the state of relations with its neighbours is also cited, although Slovenia's non-membership leaves Hungary geographically detached from the other allies.

Unofficially, the US side expresses sympathy for Slovenia, but hints that a pact between France and Germany under which they would support both or neither, left the US without the option of backing Slovenia.

For the moment, US officials believe emotional support, especially for Poland and the Czech Republic, derived largely from the sense that they were betrayed at Munich and at Yalta, will ensure that they, and Hungary, will be admitted to Nato. Adding Slovenia and especially Romania, however, could jeopardise US Senate ratification for the others. This is the argument Europe's advocates of faster expansion will have to accept at Madrid.