Despite the mental images that the phrase "Butt wars" conjures up, it is actually a very innocent trampoline game.

Object of the game To be the last man/woman/child on the trampoline standing
Number of players 2 or more; you can actually play this game with as many people that you can fit on your trampoline safely without it collapsing or people knocking each other's teeth out with various appendages
Beginning play To begin a game of Butt Wars all participants must be standing on the trampoline. One person starts to jump and begins the lead-in chant;
One..., two...,three..., four....I declare a Butt War
Everyone else joins in while jumping. On the last jump after the phrase "Butt War" is uttered everyone lands on their butt with their legs crossed.
Continuing Play After you land on your bum with legs crossed you must bounce back up on your feet to continue (you cannot use your hands to help you). Once on your feet again you are allowed
ONE bounce, then you land on your butt again with legs crossed. The cycle is repeated over and over again until only one person is able to bounce back to their feet. Everyone else is left stuck on their bums.

Most people can't accept one defeat as the determination of the ultimate Butt War champion, so one often gets wrangled into multiple Butt Battles. My sisters and I would always play best 2 out of 3. Of course the losers would then cry "Best three out of five" and it would continue forever