Complex sentence: In the event the British were obliged to participate in the French offensives further south and large British offensive operations in Flanders were not possible in 1915, due to the consequent lack of resources.
Split into simpler sentences: In the event the British were obliged to participate in the French offensives further south. Thus large British offensive operations in Flanders were not possible in 1915, due to the consequent lack of resources.