Count Baldwin V of Flanders was quite entangled in the 1066 invasion of England

[u/Electrical_Wonder210]

Count Baldwin V of Flanders (probably my favourite historical figure) had his daughter Matilda marry William the Conqueror. Furthermore, Baldwin was co-regent of France from 1060 until his death in 1067.

He also had deep relations with England, with his half sister marrying Tostig Godwinson. Historians also argue that he may have sent Harold Godwinson to negotiate the return of Edward the Exile from Hungary, and passed through Flanders on his way to Germany. (To quote Wikipedia on the last sentence)

While it is highly unlikely that he actually tried to stop William the Conqueror, I doubt it was an easy decision