Complex sentence: In the summer campaign of 1673, Louis turned back to the Dutch front and besieged Maastricht, a huge fort on the Meuse and an impediment to his operations in the Netherlands.
Split into simpler sentences: In the summer campaign of 1673, Louis turned back to the Dutch front to again attempt to deal with the fortress at Maastricht. The existence of the huge fort on the Meuse was an impediment to his operations in the Netherlands.