Complex sentence: Western Germany had been a major theatre of the Seven Years War since 1757 when the French had launched an invasion of Hanover which culminated in the Convention of Klosterzeven and the occupation of large areas of Hanover and neighbouring states.
Split into simpler sentences: Western Germany had been a major theatre of the Seven Years War since 1757, when the French had launched an invasion of Hanover. This culminated in the Convention of Klosterzeven and the occupation of large areas of Hanover and neighbouring states.