Futurama Episodes

Bender and Fry volunteer for military duty to enjoy the benefits of the soldiers’s discount. When war breaks out later that same day, Leela too decides to join the all-male ranks, under the guise of Lee Lemon, and creates quite a stir for her leader, Zapp Brannigan.
wikipedia.org summary:
Upon enrolling in the Earth army to get a military discount at the convenience store, Fry and Bender are whisked into war against a planet of ball-like aliens. Leela secretly enters the ranks to keep an eye on her friends.
imdb.com summary:
Fry and Bender enlist in the army to get a soldiers discount for bubblegum. When they are called to fight a war on a planet of bouncing ball aliens, Liela disguises herself as man to enlist with them. Bender is eventually sent to parley with the enemy, but this turns out to be a ploy.