Complex sentence: Both Abbott Thayer in the First World War and Hugh Cott in the Second World War proposed and demonstrated countershading without succeeding in persuading their armed forces to adopt the technique, though both men influenced military adoption of camouflage in general.
Split into simpler sentences: Both Abbott Thayer in the First World War and Hugh Cott in the Second World War proposed countershading to their countries' armed forces. They each demonstrated the effectiveness of countershading, without succeeding in persuading their armed forces to adopt the technique, though they influenced military adoption of camouflage in general.