How the world has changed. The Association of South-East Asian Nations, or Asean, set up in the heyday of the "domino theory" as a bulwark against Communism, has just admitted Vietnam as its seventh member. Cambodia and Laos, the other two members of what was once Communist Indochina, are expected to join Asean soon, along with Burma. Where US military might failed, the power of the market has triumphed.

Expansion of Asean to a 10-member grouping is causing some problems familiar in Europe. One or two of the core members are nervous that the organisation may be about to lose its cohesion and identity; with the addition of Vietnam's 72 million people, Asean's combined population of 420 million already exceeds that of Europe or North America. Adding another two or three countries as impoverished as Vietnam poses grave difficulties of economic integration.

But Asean's search for an economic role has always looked half-hearted. Its members tend to compete against each other with the same goods in the same markets, and to solicit investment from the same sources. What Asean has done well is to create the political conditions for its members to make money in a region that was one of the world's most unstable when the organisation was founded, and that remains capable of eruption again, despite the huge economic advance that has taken place.

South-East Asia has always been overshadowed by China, whose political and military ambitions seem to be expanding along with its wealth and self-confidence. Peking's claims to the whole of the South China Sea put it in conflict with four of Asean's seven members - including Vietnam, which has had several armed clashes and one brief war with its giant neighbour in recent years - and have ensured that much of the region's earnings are spent on arms. The attraction of Vietnam to the rest of Asean is its military weight and experience, which help to offset its economic disadvantages.

Warren Christopher, who opens the first American embassy in Hanoi this week, should appreciate the value of Vietnam in Asean as a counterweight to China, with which American relations are at their worst for some time. But the organisation's usefulness to the US and the West goes beyond that. Tomorrow Mr Christopher will attend the Asean regional forum, which brings together all the international and regional powers to discuss the kind of foreign policy and security issues Nato has existed to deal with in Europe for half a century. That is a vital function in Asia, where nuclear mavericks such as North Korea may tempt others to emulate them, and the closest thing to collective security is a fraying network of bilateral relations with the US. Against this uncertain background there is a fine irony in the fact that the addition of Vietnam to Asean's membership should be such good news.