Complex sentence: The British theatre of operations was in France and Flanders but in February 1916, Haig accepted Joffre's plan for a combined attack astride the Somme river, around 1 July and in April, the British Cabinet accepted the necessity of an offensive in France.
Split into simpler sentences: The British theatre of operations was in France and Flanders but in February 1916, Haig accepted Joffre's plan for a combined attack astride the Somme river, around 1 July. In April, the British Cabinet accepted the necessity of an offensive in France.