Real-world letters of last resort are given to the commanders of nuclear submarines, in case of the loss of national command. A few years ago, a prompt was made on the Writing Prompts subreddit to make a fictional set of orders.
To: Ship’s Commander
Subj: Last Resort
If you are reading this, national command has failed. As such, this may be the last order you receive from His Majesty and the government. This means that, until you are able to determine otherwise, you are the lawful government of the nation.
If the government has indeed failed, there is no consequence to failure to obey. Therefore, your actions must be guided by your conscience, your knowledge of international law and the Laws of War, and these written guidelines.
If a senior commander of His Majesty’s military is available, you are to report to that commander immediately. As with a fully functioning government, the senior commander is His Majesty’s representative, and his orders are law.
If an allied senior commander is available, contact them. They may have been able to establish communications with His Majesty or his representative. Allied commanders are not His Majesty’s representatives, but they may be his messengers.
A list of allied governments, in order of precedence, was issued prior to your departure from His Majesty’s domains. Seek guidance from the senior existent government. If you are unable to establish a chain of command with His Majesty, you are hereby transferred to the command of the senior government.
If you are unable to establish any form of command, or if you are the senior available officer, you are to prosecute the current action to the best of your ability in order to restore His Majesty’s or his lawful successor to the throne. If you are unable to restore the line of succession, establish a safe haven for His Majesty’s subjects. Use of all available weapon systems is authorised. Rules of Engagement package is unlocked, all ROE are granted.
Sir I.M. Boss
Prime Minister to His Majesty