Complex sentence: Based in Carrickfergus, Antrim began raising his army from December 1638 although it wasn't until April the following year that he formally received a commission from the King authorising him to do so.
Split into simpler sentences: Based in Carrickfergus, Antrim began raising his army from December 1638. It wasn't until April the following year that he formally received a commission from the King authorising him to do so.