While arms races have existed for a long time, this effort focuses only on those of the twentieth and twenty-first centuries. Since arms races constitute a critical component of foreign policy, they need a comprehensive examination. This effort provides the reasons for arms races and encapsulates historical tendencies of participants. It also explores the role of technological and economic capabilities, as well as the need for alliances. It also documents the military lead times gained by nations that first introduced a type of weapon system and explains the significance of these lead times as well. Furthermore, the responses to the leads by adversaries and potential enemies are enumerated.