REIMS, France (Reuters/AFP) - The leaders of France and Germany joined in a symbolic celebration of unity on Sunday, hailing a relationship that has brought peace to Europe for 50 years but must now prove it can survive its most serious crisis to date.Taking a pause from more than two years of constant crisis management that has strained Franco-German ties, President Francois Hollande welcomed his German counterpart, Chancellor Angela Merkel, to the city of Reims in eastern France.The event replicated an encounter in 1962 between former French president Charles de Gaulle and German Chancellor Konrad Adenauer, when the two leaders enshrined Franco-German ties in a city that once bore the brunt of shelling in World War One.But while De Gaulle and Adenauer's encounter sealed a reconciliation after two wars, Sunday's meeting was held against a backdrop of acrimony over how to handle Europe's debt crisis. "The European Union is going through a crisis - it won't be the first, it won't be the last," Hollande said in a speech by the giant double-doors of Reims' Notre Dame Cathedral, where French kings were once crowned. "But ... it forces us to move toward a new phase of development."On the eve of the watershed event, the graves of 51 World War I German soldiers were found desecrated at a military cemetery some 40 kilometres (25 miles) east of Reims.A local prosecutor said the grave markers had probably been kicked out at the Saint-Etienne-a-Arnes cemetery, which contains the graves of some 12,000 World War I soldiers - the majority of them German.French Interior Minister Manuel Valls strongly condemned the vandalism saying: "An enquiry is under way and all means are being employed to find those responsible for this terrible desecration."According to initial information, the wooden crosses had been pulled up and some used for a camp fire. Several beer bottles were found nearby.It was not immediately possible to say whether this was a "determined action" or just the work of "irresponsible people", a spokesman at the local prefecture said, adding there were no signs of any political message.Reims was occupied by the Prussians in 1870, devastated by bombings during World War I, and was the city where on May 7, 1945, US general Dwight D. Eisenhower and the Allies received the unconditional surrender of the German Wehrmacht.