Throughout his work, Guderian showed an aptitude for military theory, and in 1937, he put some of it into print in a book named Achtung! Panzer! In Achtung! Panzer!, Guderian assessed the state of armored warfare among the nations of Europe and the Soviet Union, based on his extensive studies. He argued that the era of cavalry was over due to the impact of machine-guns, and that mechanized infantry could be used to fill their role. He also set out his views with respect to the best way to conduct combined armored and armed warfare. Guderian's approach as set out in Achtung! Panzer! was one of warfare by maneuver, in which speed and surprise were essential to victory.