"No nation can build a safe, better world alone." This comes from the post 9/11 US National Security Strategy and remains true today. The threats of the world are bigger than any country can handle on their own and leaving the UN would only make the US more at risk than ever before. The UN is designed to help maintain peace and provide relief in a tumultuous global world. The US retreating into isolationism will only worsen the existing problems we see today.