Officially, the invasion was a direct response to the Attack on the Elleseandi Embassy. The true reason or cause of the attack was addressed by Albaste Bancio, who acted as Duke following the death of Duke Bizzio Pardo during the Siege.
Later revelations, such as the retelling of the story of Cassandra, suggest that there was far more to it than that and that Val Aregis was in fact a co-conspirator in the whole affair.