The 17th of September marks Constitution Day - the 208th anniversary of the signing of the Constitution. Usually it passes unnoticed. Yet it is the anniversary of the founding of what is great in America - far more than the Fourth of July with all its hoopla. We Americans would do ourselves well to give more attention to this part of our tradition. What was special in America is not that we separated, but that we built an effective Union among the new states. The point of the Constitutional Convention of 1787 was to work out a new, ''more perfect'' form of federal union, with an elected central government covering a large territory, which enabled it to manage problems competently and maintain pluralism and moderation. Such an ''extended democracy'' had never been tried before. It is what allowed democracy to work well in America - confounding 2,000 years of political wisdom, which saw democracy as a synonym for bad government, something found only in small, unstable city-states. Indeed, it is what enabled democracy to become the political ideal of the modern world. We should celebrate Sept. 17 as a true national day. That day in 1787 saw the completion of a quiet charter of inclusion and self-governance. Americans today may neglect it, but for George Washington and Benjamin Franklin, it was the culmination of their life's work. When independence was won in 1783, Washington wrote that the whole significance of independence for good or evil depended on whether the Union would be consolidated. When the Constitution was signed on Sept. 17, 1787, Franklin said he could finally put aside his fears for the fate of his country: ''Now at length I have the happiness to know that it is a rising and not a setting sun.'' He fired off a letter to a friend in Paris, urging upon him the American example - that is, urging upon him a federal union of Europe. The American founders ranked the Union highest among their achievements. They mostly viewed independence not as an inherent virtue, but as a regrettable necessity, forced by Britain's failure to reform the empire along the federal lines that Franklin had long proposed. When we have exported the ideals of constitutional democracy and federalism, as we did for much of Europe after 1945, it has worked. Our greatest successes in foreign policy - the Marshall Plan, NATO, the support for European unification - were inspired by the federalism of the Constitution, the appreciation of which had been revived by the public movements for international federation that had sprung up in the World War II period. Ignoring the Union, we do not honor the Founding Fathers; we turn their values upside down and lower them to the level of revolutionary hotheads. We need to remember their real values - the combining of self-rule with shared rule, the invention of the modern democratic form of federation, and the building of links among peoples whose fates are intertwined with one another. These same values are needed in today's world of collapsing empires and expanding interdependence.