Bug: Forced Peace?

[Truthy]

Bismarck made a pact of friendship with me. The next turn I invaded him and surrounded Berlin, his only city. Before I could take the city the game forced me out of his land, declaring "You have made peace with Bismarck". No peace agreements were met and Bismarck didn't even show up. I invaded him four more times, with the same result each time. Any possible reasons why this could have happened? I suspected a glitch occurred due to the status as "friends", just before the war.