Why Did America and the Russians Build So Many Nuclear Weapons?

Nuclear weapons are a deep threat to another country that might be threatening us. I think they do deter, and it's evident if you look at the gradual rise of the destructiveness of war from the 18th century to the middle of the Second World War. Most people killed in one year during any war in history was in 1943 when not only were combatants being killed but also the Holocaust was going on too. To kill one or two million people a year in war worldwide, we lose about six million people ayear in the world by smoking.