On July 17th of 1915, Winston Churchill wrote the following letter, sealed it in an envelope marked, "To be sent to Mrs Churchill in the event of my death," and then rejoined the army. Luckily, his wife, Clementine, never had reason to open it.
Churchill became Prime Minister 25 years later.
(Source: University of Cambridge; Image via.)
Do not grieve for me too much. I am a spirit confident of my rights. Death is only an incident & not the most important which happens to us in this state of being. On the whole, especially since I met you my darling I have been happy, & you have taught me how noble a woman's heart can be. If there is anywhere else I shall be on the look out for you. Meanwhile look forward, feel free, rejoice in life, cherish the children, guard my memory. God bless you.