An increasing number of radical right populist parties (RRPPs) got closer to power in the last few decades. Among the many available strategies to deal with RRPPs, Belgian mainstream parties have proven to be ardent advocates of disengagement through a cordon sanitaire. This chapter suggests that Belgian mainstream parties are strongly defending the cordon sanitaire principle, based on three main types of arguments: value-based, ideology-based and association-based arguments. It indicates that the cordon sanitaire faces great challenges that lead to questions about the durability of the cordon sanitaire in Belgium over time and, more generally, the durability of disengagement strategies toward RRPPs. The chapter apprehends how mainstream parties behave regarding RRPPs in Belgium, and particularly regarding Vlaams Belang (VB), and how their strategies have evolved over time. Several attempts to involve the VB in the formation of a government coalition have been made for more than 15 years.