 Did you know, Britain's nuclear deterrence is reliant on just one letter? Well technically is actually four identical letters given to the captains of the four vanguard class ballistic missile submarines. The letters which are written by whoever is the prime minister of the UK this week is thought to have won the following statements. Retaliate with nuclear weapons, not retaliate, to use their own judgement or place the submarine under an allied country's command if possible. Immediately before writing the letter the PM is explained to what damage a Trident missile could cause. The letter is stored inside a safe box that only the submarine's captain has access to. Interestingly no one apart from the writer will ever know what each prime minister has written in the letter as after they have served their time it is destroyed. It kind of makes you wonder what each prime minister has actually written and whether it or doesn't follow their public stance. Let's hope we'll never need to open that envelope in anger.