Complex sentence: Prior to the war, he established his reputation designing country houses for wealthy patrons but the war had a profound effect on him and from 1917 onwards, he dedicated much of his time to memorialising the casualties.
Split into simpler sentences: Prior to the war, he established his reputation designing country houses for wealthy patrons, but the war had a profound effect on him. From 1917 onwards, he dedicated much of his time to memorialising the casualties.