The three main reasons for the arms race were fear of future war between the sides, leaders wanting to appear strong to their own people and others, and more weapons leading to more jobs which was important for both economies. Both sides feared another world war and wanted to ensure they could win, while building weapons also supported employment, especially in the military industries, and allowed leaders to portray themselves as strong. Nuclear weapons were the main concern during the Cold War due to their ability to kill millions and potentially end the world, and both the US and USSR had them.