Complex sentence: As an ecclesiastical principality, the Abbey of St. Gallen was to constitute an important territorial state and a major regional power in Northern Switzerland up to 1803 when it lost its independence and was incorporated into the new Canton of St. Gallen.
Split into simpler sentences: As an ecclesiastical principality, the Abbey of St. Gallen was to constitute an important territorial state and a major regional power in Northern Switzerland. However, in 1803 it lost its independence and was incorporated into the new Canton of St. Gallen.