Why the 1300s? That’s generally around the time when the practice of a Country A sending people to Country B as representatives/diplomats started. Apparently these representatives are also called “honorable spies”. If you allow me to send diplomats to your country, some of which will be spies *wink*, I’ll allow you to setup your embassy in my country, knowing full well that you will also plant spies *wink*. May the best spies win.
Theatricality of modern diplomacy aside, for this system to work, countries need to have a secure way of communicating with their embassies abroad. So they encrypted their communications. This increase in enciphering naturally led to increased interest in breaking the ciphers that rivals are using to communicate. A lot of western countries thus had systems in place for setting up secure communications as well as groups that would attempt to break other nations’ ciphers.
Chief among the types of ciphers used is the nomenclator. A nomenclator is a cipher alphabet with mappings for some combination of letters, syllables and common words, names or places. Countries would create copies of the nomenclator they would use and send them with their representatives abroad.
People got really creative with their cipher techniques. They had alphabets where a given plain text could be enciphered in more than one way (homophones). They had alphabets with nulls; these are cipher text symbols that don’t actually stand for any plain text character. They got rid of a word boundaries and punctuation. They got rid of repeated letters as long as it was unambiguous. Anything to make a cryptanalyst’s job a living hell.