Complex sentence: This impasse reinforced the French and British commanders' belief that to end the stalemate they needed a breakthrough; while this desire may have been the main impetus behind the offensive, the timing and location were heavily influenced by a number of political and tactical factors.
Split into simpler sentences: This impasse reinforced the French and British commanders' belief that to end the stalemate they needed a breakthrough. However, while this desire may have been the main impetus behind the offensive, the timing and location were heavily influenced by a number of political and tactical factors.