This article tries to explain how and why the Metal Slug series was created.
Introduction: Irem Corporation
Irem was a video-game company founded in 1974, initially under the name IPM. They specialized in manufacturing arcade cabinets, later changing their name to Irem Corporation, Irem standing for International Rental Electronics Machines. In the mid 80's, they changed the abbreviation to mean Innovations in Recreational Electronic Media. During this time, they became well known for their R-Type series.
From Irem to Nazca
During the 1990s, 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 among 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 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 during this 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 to Gunforce: Battle Fire Engulfed Terror Island, an obscure run-n-gun game. Gunforce 2 was the last game made 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 their way through hordes of enemies and mechanical vehicles.
In this game you either control a male character named Max, or a female character named Lei. You use a joystick to move the character and change the gun angle, as well as two buttons to fire the guns and jump.
Many of Gunforce 2 's elements are borrowed by Metal Slug. You have a variety of weapons to pick up, you can ride a good amount of vehicles such as tanks, jeeps, walker robots and a hovercraft. Your mission is to rescue prisoners, although here they are women instead of bearded, starving POW's. Both games use very similar graphic designs, like the explosion effects for example. Some of the sound effects are the same as in Metal Slug, such as the screams of the enemy grunts, which are identical to those of the grunts hired 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 now continue to create the Metal Slug games under SNK's name.