Why would sending a letter be considered treason?

Sending a letter could be considered treason if it contains communications that betray one's nation, such as revealing sensitive information to an enemy or inciting rebellion against the government. In historical contexts, letters that conspire against state authority or aid an adversary could be seen as actions that undermine national security. Such acts can be classified as treasonous because they threaten the integrity and stability of the state. Ultimately, the classification of treason depends on the content of the letter and the intent behind it.