It’s always useful to know which countries were allied. In 1066 for example Flanders had an important role to play in the conquest. Count Baldwin V ‘s daughter was Matilda, the wife of William the Conqueror. Baldwin’s family, like the rest of Europe’s political leaders were strategically allied. Interestingly there was even a member of the family married into Earl Godwin’s family. Baldwin’s sister, Judith, was married to Tostig – Harold’s earstwhile brother. On one hand Count Baldwin allowed his brother-in-law to recruit men on the other if his son-in-law successfully invaded the kingdom that Tostig also sought he would be very influential indeed– especially as he held the regency of Philip I of France as well. (Baldwin’s wife was the daughter of Robert II of France as well as being a wily politician in his own right. He was Count of Maine as well as Count Flanders.)
William of Malmsebury indicates that Baldwin advised Wiliam and offered troops. Another chronicler notes that Baldwin was a canny neogitaor and demanded to know what benefits he would reap from his support. Allegedly William sent Baldwin a blank sheet of parchment. Wace suggests that Baldwin fed King Harold and the rest of the Godwinson Clan false information about the number of men at his son-in-law’s disposal. Family loyalty had its place but financial incentive and power were much more important.
Whilst the words of chroniclers are not always reliable, it is true to say that the Norman conquest was not entirely a Norman achievement . There was a good sprinkling of Flanders in the ranks at Hastings; the alliterative Gilbert of Ghent who later turned up in York and also Walter Bec to name but two. The Domesday Book of 1086 gives historians an insight into the role of Flemings in William’s army and the rewards that they received in the aftermath of the conquest. In total in Derbyshire Gilbert of Ghent had three land holdings whilst there were 53 Flemish landholdings in Yorkshire held predominantly by Dreux of Beuviere (George).
There weren’t just Flemings in William’s army. There were many men from Brittany in William’s army. Eustace II of Boulogne also fought for the Duke of Normandy. This is interesting as Bolougne and Flanders were rivals. The Godwinsons had marital links with Flanders through Tostig’s marriage to Judith of Flanders. Eustace on the other hand was the former brother-in-law of Edward the Confessor – his first wife Goda being Edward’s sister. His step son also had a personal vendetta against Sweyn Godwinson – Earl Godwin had famously refused to punish the people of Dover for not providing free bed and board to Eustace and his entourage when the Count was on his way home from visiting Edward the Confessor in 1051. This had become one of the flash points which led to the exile of Earl Godwin.
Looking beyond the immediate Conquest years the role of Flanders remained an important one – it was to Flanders that William’s son Robert fled when he rebelled against his father in 1077. By that time Baldwin’s son Robert I was the Count having usurped the position from his young nephew Arnulf – it was a situation that would end in armed conflict and the death of Arnulf. (Baldwin died in 1067 and was succeeded by his eldest son also called Baldwin but he died three years later leaving a young son. He asked Robert to be a regent for the boy but Robert ignored his brother’s wishes and claimed Boulogne for himself.)
The impact of Robert’s usurpation on Norman- Flemish ties is evident by the fact that Robert Curthose found sanctuary with his uncle. By 1085 Robert of Flanders had become involved in a plan to invade England and snatch the Crown from the head of his increasingly chubby brother-in-law. Robert planned to assist his son-in-law Cnut IV of Denmark who was the great nephew of England’s King Cnut.
In his own turn William had offered help to Arnulf’s younger brother (another Baldwin.)
https://sourcebooks.fordham.edu/medlow.asp
George, R. H. “The Contribution of Flanders to the Conquest of England (1065-1086)”