Why would this screen come up for me to have no option but to accept peace (that I did not start).

The screen said: "Our war with (faction) has not gone well. Meddling or helpful, depending on our point of view, diplomats have negotiated a cease fire on your behalf." At bottom of screen I had to choose "Accept the terms." 

I did not want to accept the terms. I ended loosing the game to a prestige victory. I was at war with the faction and just reached their borders with better ships. But I had to accept peace! I did not like having no other option but accept peace to continue with the game at that point (turn 155). What brought screen this up?  I play on incredible, huge with 24 factions. Any explanation would be helpful. Thanks