Source: Wikipedia, "Ozma Wars", available under the CC-BY-SA License.
Ozma Wars is a 1979 scrolling shooter arcade game, which is one of the earliest titled developed and published by SNK. In fact, at the time of its creation the company was still known as "Shin Nihon Kikaku". The game is also known as the second ever vertical shoot 'em up game, after Taito's Space Invaders (which ran on the same arcade hardware), but is also additionally known as the first game with disparate "levels". The game is also notable for being the first action game to feature a supply of energy, resembling a life bar, a mechanic that has now become common in the majority of modern action games. The game allowed the player to refuel energy between each level, and it featured a large variety of alien enemies. Many of the game's concepts would later be further developed in Sega's Astro Blaster, Sierra On-Line's Threshold, and Activision's Megamania.
The player controls a space ship which most fend off UFO's, meteors, and comets. Instead of lives, the player is given an energy reserve that is constantly decrementing; getting hit by the enemy causes gameplay to stop momentarily while a large amount of energy is depleted. Every so often, a mothership will appear and dock with the player's spacecraft, causing the energy to be refilled. There are 3-4 recognizable "stages" as the game progresses and new enemies begin to appear. After these, the re-filling ship will appear, and the cycle will start over; this occurs indefinitely.
There are two known bootleg versions of this game called Space Phantoms and Solar Flight. In Space Phantoms the player's ship looks like an angel, and the enemies appear as different types of insects. Due to the game being monochrome and a conversion kit for Space Invaders, many Ozma Wars monitors still utilized the Space Invaders color overlay.