Complex sentence: The Istrian Circle (:) was a province of the Kingdom of Illyria from 1825 until 1849, when Illyria was reincorporated into the Austrian Empire and Istria returned to being a crownland ("Kronland") of Austria as the March of Istria.
Split into simpler sentences: The Istrian Circle (:) was a province of the Kingdom of Illyria from 1825 until 1849. In 1849 Illyria was reincorporated into the Austrian Empire and Istria returned to being a crownland ("Kronland") of Austria as the March of Istria.