LEADING ARTICLE : Keeping the iron curtain drawn

Two events in two days underline how delicately poised East-West relations have become. Nato's Secretary-General, Javier Solana, has begun a tour of the eastern European countries, many of which want to join Nato. This is not a formality; it could have far-reaching political signifiacnce.

That is because of an event that will take place today: the close for nominations for the Russian presidential elections in June. As things stand, those elections are likely to be won by the Communist leader Gennady Zyuganov. The prospect of several eastern European states joining Nato is the kind of development Mr Zyuganov will use to inflame the resentful Russian nationalism that is at the core of his appeal. A spectre that we might have thought had been banished may re-emerge: a Europe divided into east and west, albeit with the line slightly to the east of the old one, closer to Russia.

Mr Solana will tell candidates for Nato membership that, in spite of Russian objections, they have a right to join as sovereign states, and that they should eventually do so in the interest of wider European stability and security. No country - including the Baltic States that abut directly onto Russia - is ruled out. The first wave - Poland, Hungary and the Czech Republic - might accede to Nato before the end of the century.

These will be difficult promises to keep. There are three, related problems: the risk of alienating Russia; the risk of a new iron curtain forming; and the dilemma of what to do about the countries that might lie between an expanded Nato and newly isolationist Russia.

Nato was created as a shield against the power of the Soviet Union. Russia will regard any Nato expansion eastwards as a hostile act. Although it may be in turmoil, Russia is still a mighty force. Its armed forces may no longer match those of Nato, yet it is still a nuclear superpower and it has more conventional military power than any other state.

Whatever Nato members feel about expansion, Russia will not regard it as benign without cast-iron guarantees that would reduce Nato's effectiveness as a military alliance. It might be possible to write in prohibitions on the movement of troops and equipment into east European countries. But unless it was possible to deploy forces into those countries if they were threatened or attacked, the Nato security guarantee would mean nothing.

So is there an alternative to Nato expansion? In a near-perfect world, European security and stability could be equally well served by the more prosperous east European states joining the European Union. Yet a prosperous member of the EU adjacent to a crime-ridden, anarchic or autocratic state could hardly feel secure without a security guarantee. That may be an argument for expanding the EU's role in security. Economic integration will depend on some form of security guarantee.

We are moving away from an era where two opposed, cohesive structures could provide the security structure for Europe. In future - to borrow a phrase from another European debate - we will need a security system based on variable geometry of overlapping bi-lateral and multi-lateral economic and security arrangements.