By the end of the war, the modern "empty battlefield" was evolving, with troops learning to avoid open spaces, and the light field gun was becoming obsolete, with an increasing use of light machine guns, light mortars and field howitzers which, with their high trajectory, were able to drop shells onto even deeply sheltered enemy troops on reverse slopes that field guns could not reach. The 18-pounder Mk IV, with its box trail which allowed it to fire in a high trajectory, had begun its evolution into the more versatile 25-pounder gun-howitzer.