He was attending a celebration marking the 50th anniversary of the Treaty of Rome, which had set the foundations of the modern European Union.
"Europe's unification is a millennium endeavour. For the first time Europe has united on the basis of the interests of those big and those small, for the first time it is uniting all its potentials, becoming a real partner to the United States and the continent on which everything that happens in the world in the future will depend," Mesic told Croatian reporters.
"United Europe first of all rules out war as a political means, cancels motives for the forcible changing of borders and transforms national minorities into connecting bridges," he said.
"Croatia can be important for Europe economically, but one must not forget that it can also enrich it with its cultural, scientific and other potentials as well as with its historical heritage."
"It is up to Croatia to adopt European standards and meet the conditions, and it is up to Europe to recognise that," Mesic said, adding that Croatia expected to be ready for EU membership by the end of 2008 so that it could join the year after.
Mesic attended a concert by the Luxembourg Philharmonic at the Bozar Centre for Fine Arts at which Belgian Prime Minister Guy Verhofstadt and European Commission President Jose Manuel Barroso delivered speeched.
Mesic also attended a symposium on the importance of European integration organised by King Albert II of Belgium.
Also attending the Brussels celebration were presidents Toomas Hendrik Ilves of Estonia, Laszlo Solyom of Hungary, Edward Fenech Adami of Malta, Valdas Adamkus of Lithuania and Branko Crvenkovski of Macedonia, Bulgarian Vice President Angel Marin, Queen Beatrix of the Netherlands and Henri, Grand Duke of Luxembourg.