His father arranged his marriage, under threat of arms, to Richilde, Countess of Hainaut. She was the widow of Herman of Mons and heiress of Hainaut. Because Hainaut was a part of the Holy Roman Empire this enraged Emperor Henry III who had not been consulted. He went to war against the two Baldwins but was not successful. Between 1050 and 1054 Lambert II, Count of Lens fought alongside the Baldwins against Henry III. He was one of Baldwin V's leading vassals. Lambert was killed at the battle of Lille in 1054. Others fighting for the Counts of Flanders against Henry III included the lords of Alost.