The United States is considered the first modern federation. After declaring independence from Britain, the U.S. adopted its first constitution, the Articles of Confederation in 1781. This was the first step towards federalism by establishing the federal Congress. However, Congress was limited as to its ability to pursue economic, military, and judiciary reform. In 1787, the Second Continental Congress drafted the United States Constitution during the Philadelphia Convention. After the ratification of the Constitution by nine states in 1788, the U.S. was officially a federation.
Source: Wikipedia, 2011