WASHINGTON—At midnight on Monday, Croatia became the 28th member state of the European Union. After the long lull following the addition of ten new members in 2004 and the subsequent joining of Bulgaria and Romania in 2007, this latest enlargement of the EU occurs during the protracted eurozone crisis and amid much talk of “enlargement fatigue.”

A number of other momentous decisions were made last week at the meeting of the Council of the European Union. Serbia was given a green light to begin EU accession talks in January 2014. Kosovo is to begin talks on a Stabilization and Association Agreement, a direct result of the historical agreement that Belgrade and Pristina achieved in April following the successful mediation efforts of EU High Representative Catherine Ashton. European leaders also endorsed Latvia’s bid to join the common European currency as the 18th member of the eurozone. The significance of this moment cannot be overestimated.

Enlargement has been one of the EU’s greatest successes. The making of a Europe whole, free, democratic and at peace was the righting of post-World War II wrongs, which had left many countries under the Soviet yoke. This past month marked the twentieth anniversary of the Copenhagen criteria, which set down the conditions for post-communist countries’ accession to the EU: put simply, democracy, human rights, the rule of law, and a market economy. In addition, it was ten years ago at the Thessaloniki Summit that the EU opened the door wide open for the countries of the Western Balkans to become members.

Yet in certain EU countries and in parts of the EU establishment there has been growing skepticism about adding new member states. Some point to the supposedly “premature” accession of Romania and Bulgaria, the entry of Cyprus with its unresolved territorial issue, the challenge of membership negotiations with Turkey, and the global economic crisis of 2008.

The Western Balkans offered further complications. With the breakdown of Yugoslavia in 1991, the EU thought in the infamous words of Jacques Poos that the hour of Europe had dawned and that it could manage the crisis. To no avail. It took nearly a decade of war, bloodshed, devastation, and fragmentation for peace to be achieved. Although most Europeans have no memory of war and violent conflict, the polities and populations of the Western Balkans have had the misfortune of experiencing war and destruction only a decade ago. This is what in large part drives their desire to join the European Union.

It is to their credit that Brussels and major EU leaders never reneged on their 2003 promise of membership for the Western Balkans. Thus the soft power of the EU remained effective throughout and showed its effectiveness despite skepticism and the fact that domestic political conditions and the economic crisis often overshadowed altruistic European goals.

The process of accession has become much more rigorous with each new enlargement. Croatia and the countries that are following in its footsteps have had to make significant progress in confronting past war crimes. Combating corruption and organized crime has been another key priority, as well as strengthening the rule of law through the creation of credible independent judiciaries. Croatia even had to arrest its former Prime Minister Ivo Sanader, who initiated the EU integration process.

The EU has done well to keep the door open to new members and the enlargement process alive. The Western Balkans - an area at the heart of geographical Europe - is completely surrounded by EU and NATO member states, and thus EU integration is necessary to fully stabilize peace and create sustained security. With this enlargement, the EU’s credibility has been enhanced, its soft-power vindicated, and the people and leaders in candidate countries see that their democratic reform efforts are being recognized and rewarded.

Ivan Vejvoda is vice president for programs at the German Marshall Fund of the United States in Washington, DC.