Complex sentence: During the last decade of the Thirty Years' War, the Spanish forces in the Spanish Netherlands were sandwiched between French and Dutch forces and the French won a major victory at Lens but Franco-Dutch forces could not decisively crush the embattled Army of Flanders.
Split into simpler sentences: During the last decade of the Thirty Years' War, the Spanish forces in the Spanish Netherlands were sandwiched between French and Dutch forces. The French won a major victory at Lens, but Franco-Dutch forces could not decisively crush the embattled Army of Flanders.