Complex sentence: General Douglas Haig, then commander of the British First Army, directed the battle; however, his plans were limited by the shortage of artillery ammunition which meant the preliminary bombardment, essential for success in the emerging trench warfare, was weak.
Split into simpler sentences: General Douglas Haig, then commander of the British First Army, directed the battle. However, his plans were limited by the shortage of artillery ammunition, which meant the preliminary bombardment, essential for success in the emerging trench warfare, was weak.