Buckle up. Victor OpenDev version, around turn 130:

The situation:

- I was Allied with Mauryans

- They declare war on Mycenians

- I asked for surrender of Mycenians, without going to war. They refuse.

- several turns later, I go to war with Mycenians, take one of their cities, destroy several armies of them

- On next turn I'm suppose to win.

- The Mycenians win the war against the Mauryans and vassalize them

- I lost the city I was occupying

- in the diplomatic screen, I'm allied with the Mycenians and the Mauryans

- I still can fight Mycenians but I can't make peace

- I broke Alliance with Mauryans, war's completely over

- I tried to reboot, broke the alliance with the mauryans before they was vassalized - same issue: state of war on the ground but peace on a diplomatic level...

I had a similar issue where an ally became the vassal of an enemy mid war. I was then at war with my ally, but we couldn't actually attack each other.

Seems transitions to a vassal causes issues with pre existing diplomatic relations.

Hi there!

Do you happen to have a save both just before the issue and with the issue occurring?

Save files by default are located: Users\[user]\Documents\Humankind\Save Files