Now if i remember correctly, reflecting back to what i remember from my grade 9 history, the Nagasaki and Hiroshima atomic bombs of WWII were one of the early bombs so they were small compared to how destructive they were eventually able to make them. The better ones were something like 20 times as powerful as the ones detonated on Japan (again i'm ballparking the numbers but you get the idea). Now the interesting part was that eventually they designed the H-bomb (or the Hydrogen Bomb) which was several thousand magnitudes more powerful than any atomic bomb. So yeah... imagine the destructive power we are capable of today.
During the Cold War we apparantly had enough nuclear warheads to blow up the world over 20 times over. I'll leave it with Einstein's famous quote: "I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones."