Complex sentence: The Kezdai began a campaign of expansionary conquests against Concordiat worlds during the period of total war with the Melconians, however the Kezdai vastly underestimated the actual size of the Concordiat.
Split into simpler sentences: The Kezdai began a campaign of expansionary conquests against Concordiat worlds during the period of total war with the Melconians. However, the Kezdai vastly underestimated the actual size of the Concordiat.