In World War I almost all infantry walked over the battlefield on foot. Likewise artillery was still towed by horses. As automobiles became more widely available, tractors and trucks started to take replace the animals in some places. Mobile gun development was overtaken by that of motorized infantry, which came into being during the 1920's CE, in an attempt to make infantry more mobile than it had been in the Great War. Its full potential was realized by the German Wehrmacht in the next decade. The German motorized infantry was set up to support rapdily advancing tanks that outpaced infantry on foot. Likewise, supply columns were also equipped with motorized vehicles, so that they could keep up with the fast advancing troops.
Trucks perform well in areas with good roads, but less in sparsely populated areas where they have to plough through heavy terrain; they lack the cross-terrain capabilities of tanks. Sometimes they are armed with machine guns.
During World War II, where motorized troops played a significant role, trucks and other vehicles were still uncommon and expensive and used up precious fuel. Therefore most of the infantry still marched on foot and guns were pulled by horses. Only a few elite mobile units used trucks, though these were important in rapid offensives. Only the USA and Britain, near the end of the war, were able to motorize their infantry and artillery on a large scale. The most valued form of aid in the war from the western allies to the USSR, next to food and clothing, consisted of trucks.
Since World War II, cars and trucks have become widespread and today almost all infantry and artillery is equipped with some sort of transport. Motor vehicles improve mobility, but offer no other advantages. In many armies, motorized troops were a brief transitionary stage towards mechanized infantry and self-propelled artillery.