Complex sentence: Their campaign in Asia Minor during 1303 and 1304 was a series of military victories, but alongside widespread looting of Byzantine civilians, so when they insisted in receiving the agreed payment, the Emperor refused.
Split into simpler sentences: Their campaign in Asia Minor during 1303 and 1304 was a series of military victories, but with widespread looting of Byzantine civilians. When they insisted in receiving the agreed payment, the Emperor refused.