Despite being neutral at the start of World War II, Belgium and her colonial possessions found themselves at war after the country was invaded by German forces on 10 May 1940.he invasion of Belgium by Nazi Germany started on 10 May 1940 under the codename Fall Gelb (Case Yellow) as part of the wider invasion of France, the Netherlands and Luxembourg. The Belgian Albert Canal fortifications, some of the most modern defensive networks in Europe, proved almost useless.