[1] He subsequently made peace with Philip, who became his stepson-in-law, but remained hostile to his sister Matilda and her husband William the Conqueror, who was king of England and duke of Normandy.

[2] His elder brother, Baldwin VI, succeeded their father as count of Flanders in 1067, and his sister Matilda had married William the Conqueror, then duke of Normandy and later king of England in 1051/2.

[9] As a part of their negotiations Corbie, an important trade center, which had been ceded by Arnulf III in order to secure the king's help, was returned to royal control.

[1] From then on, and until the repudiation of Bertha in 1092 (in order for King Philip I to marry Bertrade of Montfort), Flanders and France remained in very friendly terms, both of them having the new Anglo-Norman realm as a main enemy.

To obtain funds, they enfeoffed Hainaut to the bishopric of Liege and counted with the support of Godfrey IV, Duke of Lower Lorraine.

[9][11][c] In 1075, Robert let the Danish fleet of king Sweyn II of Denmark use Flanders harbors in their intended expedition against England.

The threat was big enough for William to hire mercenaries and lay waste to some coastal districts in order to difficult the supplying of an invading army.

[9] Even before becoming count of Flanders, Robert was engaged in continued hostilities in Holland, defending the rights of his stepson Dirk V against Godfrey IV and William I, bishop of Utrecht.

[15] The victory changed the course of the hostilities, allowing Dirk and future counts to reconquer the territories of the county of Holland they had lost in the past.

[1][16][17][18] During the papacy of Urban II the Flemish clergy complained about the exactions imposed by Robert at a provincial council in Rheims (around 1092).

[1] Taking a considerable armed escort Robert the Frisian made a pilgrimage to Jerusalem in 1086 and on the return trip home spent time assisting the Byzantine Emperor (Alexios I Komnenos) against the Seljuq Turks.