This article by Erik Brattberg, co-authored with Tobias Bunde, Maksymilian Czuperski and Marietje Schaake, appeared today on the Huffington Post. After centuries of war, since 1945 and the fall of the Berlin Wall in 1989, freedom, democracy and tolerance have served as the unifying forces during the longest era of peace and prosperity on the European continent. What was once used to divide us has become the key of our success: Europe's diversity. More than half a century since the signing of the historical declaration by French foreign Minister Robert Schuman, half a billion people from 28 countries now live together in peace, united by the principle of freedom, democracy and prosperity. It is because of these achievements that we celebrated Europe Day this past weekend. But the glue that keeps us together, the values of liberal democracy, are being tested, both internally and externally. Within Europe, cracks in the European Project are evident. In the wake of the euro-crisis, populism is on the rise throughout the continent, threatening to derail or even destroy the EU as we know it. Euro-skepticism is now the glue that binds far-right and far-left parties together. Countries such as Hungary are sliding back towards semi-authoritarianism, and others may well follow suit. A growing North-South economic divide is threatening to undermine solidarity. The recent UK election and increases the risk for a "Brexit" and a potential fragmentation of the EU as we know it. Meanwhile, Europe is experiencing a perfect storm of threats in its Eastern and Southern neighborhoods. Russia's annexation of Crimea a year ago and subsequent aggression in Eastern Ukraine has called the entire European security order into question. What's more, Russian style authoritarianism has also begun to make inroads in some parts of Europe while Moscow is making new friends with far-right and far-left parties throughout the continent. To its southern flank, a wave of extremism known as ISIS / DAESH has managed to attract thousands of young European men and women to join its wave of terror, while triggering a new refugee crisis that has left millions displaced across the Middle East. In light of this plethora of challenges, some critics have been quick to declare the European Union as failed. Certainly there are challenges and obstacles that Europe has to overcome. But our track record of the past decades has just shown the potential we have: We Europeans have once and for all left the age of war and tyranny behind us and entered an era of human rights, reconciliation and prosperity. The values that unified us half a century ago still unite us today. It is those values that have become Europe's unifying fabric. It is those values that thousands of people looked, and stooped up for on the Maidan square a year ago, wrapped in the European flag and what its idea of tolerance, democracy, and freedom stands for, through bitter cold and bloody, even lethal brutality, united around the dream of living in a country based on the very principles that our union has been built on decades earlier. Thousands drown each year in the Mediterranean Sea, hoping to reach Europe, because for them it has become an island of hope and security in a world of turmoil and uncertainty. Europe has become so attractive to much of humanity that as Roger Cohen put it in the New York Times, "thousands die trying to get into it, a Continent where entitlements including universal health care are seen not as socialist indulgence but basic humanity, and a magnet to states outside the European Union that long to be part of this security-conferring entity." More than ever, European leaders must unite in common cause to defend the very principles this union of ours has been built on so that the privilege of being born into this age of relative stability, security and prosperity can be passed onto our children. These principles we must defend, so that Europe remains an inspiration for those parts of the world where the difference between people, tribes and ethnicities will be alike seen as the key to the future and not exerted to divide. This requires standing up for the values both within our borders, and outside our borders. We must stand together even if the effects of a financial storm keep some of our members divided. We must give our hand to those who seek our help from Ukraine to North Africa. We must recognize that we have not been born into a status but been given the tools to continue the process of perfecting this union, making it better and learning from our past mistakes. And we must never stop improving the union, understanding that the principles that have ended centuries of bloodshed on the continent that we call home today - freedom, democracy and tolerance - remains a continued work in progress. The process of European integration has been declared dead by its critics time and again. Yet, to every crisis Europe has responded with another bold initiative taking the integration process to another level. It is time for Europe's leaders to step up to the contemporary test of our liberal order. This is not the time for business as usual. It is the time for transformational leadership that takes the initiative and self-confidently defends and promotes the liberal-democratic values - both within and beyond our union. This is why we celebrated Europe Day last weekend. Erik Brattberg Senior Fellow McCain Institute for International Leadership (SWEDEN) Tobias Bunde Head Policy & Analysis Munich Security Conference (GERMANY) Maksymilian Czuperski Special Assistant to the President Atlantic Council (POLAND) Marietje Schaake Member of the European Parliament D66/ALDE (THE NETHERLANDS) The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of their designated employers.