WASHINGTON — German Foreign Minister Hans-Dietrich Genscher's cherubic nature belies his shrewd eye for world events. In the 17 years since he took office he has transformed himself from a geopolitical novice who spoke no foreign language to a seasoned diplomat with experience far outstripping Western counterparts.With Germany now unified, Eastern Europe free, and even the Soviet Union abandoning communism, Mr. Genscher might be expected to see clear sailing. Instead he says the Western democratic alliance "is now facing its third major challenge after the end of the Second World War." The first challenge, he says, was saving the shattered nations of Western Europe after 1945. Rather than repeat its post-World War I mistake of withdrawing into a shell, the US stayed engaged in Europe with the Marshall Plan - "probably the most important decision America has taken this century," according to the German Foreign Minister. The second challenge was the long-term Soviet arms buildup aimed at outracing the US and prying it away from Western Europe without actually using force. NATO's formation was the reply. A milestone was the alliance decision of the late 1970s, which called for deployment of new US nuclear missiles in Europe while negotiating their elimination with Moscow. As serious as these is the third challenge: reconstructing the political culture of Central Europe in the wake of communism's collapse. "Communism killed itself more or less," says Genscher. The danger to avoid is political vacuum. A structure needs to be built that somehow includes all of Europe. The opposite of such inclusion is what's happening today in Yugoslavia. "If we can manage this we can open an opportunity for the new millennium that will be unique," says Genscher.