At the start of the game (4000BC) the first civilization that I had met besides myself was Polynesia, and they effectively became partners with me pretty much instantly. They were also the only other civ on my relatively small continent, so all of my trade was with them until I was able to trade by ocean. By the time the Medieval era had rolled around, the Ottomans to the west of us had gone full retard and were attempting to annex Polynesian cities. Naturally, they asked me for help with this situation, seeing as the gunpowder units used by them were much more advanced than either of our militaries. We would have stood a chance together, but unfortunately I ended up doing all the work and getting all my samurai killed trying to liberate their city while Kahehameha sat back and did nothing. After a failed attempt, a rebuilding of my army and the invention of muskets, I made a second push and liberated Insert Name Of Random Polynesian City Here, returning it to its rightful owner before Suleiman's troops managed to even arrive to defend it. I was about to raze another Ottoman-owned city with my remaining guys just for effect when they negotiated a peace treaty which favored me. Figuring they learned their lesson, I accepted it, and that was the end of it until a couple hundred years later during the modern era.
By this time, the Ottomans were no longer going full retard, I had expanded and had mamaged to amass great wealth and build up a standing army, which was very powerful despite being not as technologically advanced as those of many other civs. Suddenly, one fateful turn, Isabella of Spain attempted to inform me that Polynesia was plotting against me and/or organizing a sneak attack against me. I disregarded this, believing it to be Fake News as my relationship with them had remained good for the entire game. Eventually, though, it turned out to be correct. Kamehameha made some condescending comment along the lines of "I WAS ONLY PRETENDING TO LIKE YOU BECAUSE I FELT SORRY FOR YOU, BAKA" and pouring a fucking massive number of infantry, artillery and bombers into my borders with no warning. I spent almost all of my gold on fighters planes in an effort to defend myself from the bombers, and hoped that the dakka of my ground troops was sufficient to defend the homeland from everything else, and somehow it was, with me managing to destroy nearly all of Polynesia's forces while only sustaining damage to the area surrounding one of my cities. This left Polynesia's military in a hilariously weak state, with essentially no units left to defend their own cities from retaliation. I gave them essentially the same treatment that I had intended to give Suleiman, easily managing to capturing and razing two of their closest cities before ending the war.
And now I need to find someone new to trade with since I don't trust these guys anymore. I probably should have known they were insane when they banned salt worldwide, long before Spain said anything.
Civ 5, playing as Japan
How much of a fucking douchebag do you have to be to fake friendship for 6000 years?