In Athens, Greece, origin of the Western ideals of democracy and human rights, the heads of government of twenty-five European nation states gathered to sign the Treaty of Accession into the European Union on April 16, 2003. Fifteen nations were already member states of the European Union and they had unanimously agreed to accept all ten of the accession states into their club, out of the thirteen candidate countries that had applied. But the agreement was conditional on each of the ten applicants agreeing to accept all the existing rules of the club as explained to them and interpreted by the fifteen incumbents (the so-called acquis communautaire of the European Union). Further, each of the ten applicants had to demonstrate to the satisfaction of the incumbents not only that they accepted all the acquis but that they were capable of putting it into practice on their own, without extensive subsidies or technical assistance from the existing members. Throughout the rest of the year each accession state put the treaty to a referendum of their voters, and in May 2004 the European Union expanded its size to twenty-five member states, with a population of 454.5 million and an estimated gross domestic product (GDP) of $10.3 trillion, making it the largest economy in the world, surpassing the United States of America, with its fifty states, 280.5 million people, and GDP of $10.1 trillion.