Several thousand Nazi troops following the Panzer divisions crossing their borders.
Neutral country, but a vital keystone for invasion, in the end, whose aristocratic elite supported Nazi influence, because they are surrounded by potentially hostile countries forceful stability was particularly attractive - the weight of Nazi Germany was appealing for the aristocratic/ ruling parties naturally.
They surrendered in 18 days in a bloody battle held by the general population but the ruling aristocracy were prepared for Jewish persecution and sent 1000's of Jews to concentration camps including Auschwitz - 60, 000 Belgians (aristocratic) were found guilty of collaborating with the Nazis and it is reported by Belgian historians that Belgium went far beyond Nazi requirements of collaboration in WWII especially when it came to persecution. There are stories in the later war of Belgium sending refugees into occupied Poland straight into the heart of German concentration camps.
The general population of Belgium are not to blame they provided a strong resistant party but for the aristocracy (not the King) its another matter...
N.B. The largest collection of Nazi memorabilia is held by a duke in a Northern Province of Belgium - Brabant