This article tries to explain how and why the Metal Slug series was created.
Introduction: Irem Corporation
Irem was a video-games company founded in 1974 under the name IPM. They specialised in manufacturing arcade cabinets, changing their name to Irem Corporation, Irem standing for International Rental Electronics Machines. Later in the mid 80's, they changed the abbreviation to Innovations in Recreational Electronic Media. During this time, they became well known for their R-Type series.
From Irem to Nazca
During the 1990's, a group of programmers from Irem started creating various arcade games who all shared the same graphics style and design. This group, formed by Akio, Susumu, Tomohiro and Kozo amongst others, created games such as Cosmic Cop (Armed Police Unit Gallop in Japan), Undercover Cops, In The Hunt (whose Playstation conversion was, ironically, pirated and sold under the title Metal Slug Submarine) and especially Gunforce 2. During this time, Irem was facing financial problems, which ended with Irem Corporation closing forever in 1994 (Irem would come back in 1996 as Irem Software Engineering Inc, now making only home console games). The aforementioned group of programmers, frustrated with Irem's inactivity at the time of crisis, left Irem and created their own company named Nazca. They decided to create games for the Neo-Geo MVS arcade system, starting with the golf game Top Players Golf.
From Gunforce 2 to Metal Slug
Gunforce 2 (Geostorm in Japan) was a sequel of Gunforce: Battle Fire Engulfed Terror Island, a obscure run-n-gun game. Gunforce 2 was the last game created by the Nazca staff at Irem, and also the last Irem arcade game, created in 1994, the same year they closed. It was a war-themed run-n-gun similar to Contra, where the players carried two guns and had to fight your way through hordes of enemies and mechanic vehicles.
In this game you control a male character name max or the female character name lei, you use a joystick to move chracter and change gun angler, 2 buttons for fire gun and jump.
Gunforce 2 had many elements which were borrowed by Metal Slug. You had a variety of weapons to pick, you could ride a good amount of vehicles such as tanks, jeeps, walker robots and a hovercraft. Your mission was to rescue prisoners, altough here they are women instead of bearded, starving POW's. Both games use the same graphic designs, even with similar explosion effects. Some of the sound effects are the same as Metal Slug, such as the enemies grunts, which are the same grunts used by the Rebels.
When the ex-Irem programmers created Nazca, they decided to create a new game based on Gunforce 2, thus creating Metal Slug: Super Vehicle-001. The game was a success, and SNK, the owner of the Neo-Geo MVS system, decided to assimilate the entire Nazca staff into itself, so they would continue to create the Metal Slug games are now under SNK.