Yeah, I've just had this in my current game.
I'm the English. At some point the Iroquois approach me, and we sign an MPP. Then I go to war with the Aztecs (they founded a city in the same spot where I wanted one). Two turns later the Aztecs sign an embargo against me with the Iroquois. The next turn, I sign an embargo against the Aztecs with the Iroquois, and the turn after that, the Iroquois honor their MPP with me, and attack the Aztecs.
Now if that isn't complicated
Asmodean