In Worm War I players control a vertically-scrolling tank at the bottom of the screen and must shoot Worms that move back and forth in a fixed pattern (left difficulty switch "B") or in a random pattern (left difficulty switch "A"). You must destroy them or lose valuable fuel every time you collide with one. You can control the speed of your ascent with your joystick.
You start the game with 99 units of fuel and slowly run dry as time goes on. Fuel is rapidly depleted when you collide with a worm or run into a randomly placed obstacle. You can shoot the obstacles as well as the worms. To refuel, you must roll over a Pagoda Gas Station that randlomy appears while being careful not to destroy it.
There are three basic variants for one-person play: playing with obstacles, playing with no obstacles, and playing at night when the worms only appear briefly. In two-person play, each player gets a tank on the screen at the same time. Players can play cooperative games wherein they share fuel and points, as well competitive games when each player maintains their own supplies.