Complex sentence: Gough stressed the need to occupy temporarily undefended ground, which was more likely in the first attack, with its longer preparation than later attacks; after discussions at the end of June, Haig endorsed the Fifth Army plan.
Split into simpler sentences: Gough stressed the need to plan to take ground left temporarily undefended, which was more likely in the first attack, with its longer period of preparation. After discussions at the end of June, Haig endorsed the Fifth Army plan.