KCNA via Reuters North Korea has worked for more than a decade to build nuclear weapons and missiles. Although Kim Jong Un, the isolated nation's leader, has been called "mad" and a "rocket man," a US nuclear policy expert doesn't think he's crazy. Kim Jong Un likely wants nuclear weapons to deter US invasion because he doesn't trust the country. Obtaining nuclear weapons would give North Korea a chance at diplomacy more in its favor, similar to China during the Cold War. But Kim Jong Un could also take his country in the opposite direction. North Korean leader Kim Jong Un has been called many things — crazy, mad, insane, and "rocket man" — because of his program to build nuclear bombs and missiles capable of launching the weapons to the US. But experts say he is not crazy to want a nuclear arsenal.