Serbia was already 'conquered' at the start of WWI, it was part of the Austro-Hungarian Empire but they wanted to be an independant state of united Slavs, or Yugoslavia. It was a freedon fighter by the name of Gavrilo princip of the underground group Black Hand who assasinated Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, that sparked WWI, as Serbia's ally was Russia, who were allies of France and Britain. Germany, allies of the Austro-Hungarian empire therefore declaired war on France, and a general came up with the idea of flanking the french armies by going through Belgium, named after the general, the Schleiffen Plan. Unfortunatly the belgians put up a good fight and managed to hold off the Germans until the British came to aid the Belgians because of an old treaty they had signed. Britain was never invaded during WWI, but it has been invaded many times since the norman conquest, some unsuccessful (Bonnie prince Charlie), some successful (Henry Tudor and William of Orange)