The latest wave of European Union (EU) enlargement is the most far-reaching and arguably the most significant. On May 1, the EU admitted 10 new members, a move that has unified Europe after 50 years of division. This significant achievement pushes the frontiers of a new international political order based on shared values and a desire to extend peace and prosperity across the continent.

At the end of the Cold War, the international community seemed determined to bring post-communist states back into the fold of democratic nations. In the early 1990s, the EU initiated the dual processes of deepening economic and political cooperation within the EU and widening membership to post-communist Europe. There were many barriers to success: the communist legacy left economically devastated societies that hardly seemed ready for participatory democracy. It is remarkable, therefore, that only 15 years since the fall of the Berlin Wall, eight post-communist countries have stepped up to the challenge of adopting the political and economic reforms required for EU membership.

It is important to recognize this as a pivotal milestone in the process of European integration, but not the finish line. Challenges remain for post-communist Europe and the EU as a whole. First, without the carrot of EU enlargement to compel further progress in democratic consolidation-for example in the protection of the Roma minority or the fight against corruption-the EU can do little to prevent political backsliding among its new members. Here, continued U.S. support and involvement will be necessary.

Second, the consequences of this enlargement are still unclear, which has heightened anxiety about future waves of EU enlargement. The countries next in line for membership-Bulgaria, Croatia, Romania and Turkey- not to mention those that aspire to join from the still-unstable Western Balkans, will need a strong commitment from the EU if the same transformation is to be realized there.

Third, the EU needs to adopt its long-debated draft constitution, which would introduce the institutional reforms necessary to streamline decision-making and address the EU's democratic deficit. Finally, the United States must continue to support the European Union and make an effort to better understand its limitations and capabilities so that the vital transatlantic relationship remains strong.