Testing time for Belgium

Belgium's election returns dramatize not only its unique domestic political situation but its far from unique economic situation as an advanced country facing industrial change. How Belgium copes with the decline of its pioneering steel industry, for example, could be instructive to countries much larger than one measuring 175 miles at its widest.

Small is not simple in this case. There is more to Belgium than meets the eye of those who think of it mainly as the host of NATO and the European Community. Since the 19th-century when its steel spurred the industrial revolution in Europe, Belgium has given the world lessons in prosperity. The size of one American state (Maryland), it has become the ninth biggest trading partner of the US. It gives twice as much of its GNP to aid developing nations as the US does. More than 1,200 US-owned companies are located there.

But recently economic problems have been building to the point of bringing down Belgian governments. Sunday's election had to be called after a Socialist demand for bailing out the steel industry led to the fall of the existing coalition. At this writing the Socialists appeared to have gained a little ground, overtaking the moderate, long dominant Social Christians, who had opposed the bailout. Meanwhile, they were both being crept up on by the Liberals , the private-enterprise conservatives registering strong gains.

If this sounds complicated, remember that each of these three major groups - Social Christian, Socialist, and Liberal - is actually two parties. They are linguistically divided in line with Belgium's northern Dutch-speaking Flemings and southern French-speaking Walloons. Considering the election returns, all six might have some claim on a new coalition government. But the Flemish Socialists, reflecting the antinuclear momentum in Europe, have vowed not to join a coalition with any party, such as the Social Christians, that accepts US cruise missiles in Belgium.

It all has to be seen against the background of Belgium's efforts to resolve the regional strains that have been aggravated by the advent of high unemployment, federal deficits, and other economic challenges. One small party gained several seats with a call for greater autonomy for the language communities. There could be pressure to accelerate an already mandated program for ''regionalization.''

In short, Belgium has its work cut out for it. The skills and energies that have distinguished its past will have to be vigorously called into play. Its friends and neighbors can watch and learn as well as offer good wishes.