A shorter weapon was more convenient when riding in a truck, armored personnel carrier, helicopter, or aircraft, and also when engaged in close-range combat. Based on the combat experience of World War II, the criteria used for selecting infantry weapons began to change. Unlike previous wars, which were often fought mainly from fixed lines and trenches, World War II was a highly mobile war, often fought in cities, forests, or other areas where mobility and visibility were restricted. In addition, improvements in artillery made moving infantry in open areas even less practical than it had been.