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The Strategic Defensive Initiative, or S.D.I., was a program put forward by President Reagan in the 80s as a means to defend the US from potential ICBM (Inter-Continental Ballistic Missile) attacks from enemy nations. A satellite armed with state of the art weapons technology would shoot any launched ICBMs out of the air from space. It was considered far too expensive a proposal and was pejoratively dubbed "Star Wars" by the press at the time.
This game realizes this idea, in a sense, by allowing the player to assume one of these SDI attack satellites as it takes out missiles and enemy vehicles from an unknown alien threat.
Global Defensive was originally developed for the Arcade. Sega created a home version for the Sega Master System and other publishers (like Activision) created versions for home computers.
The game is split into two modes: Offensive and Defensive.
The initial Offensive mode plays like a horizontally-scrolling shoot-em-up in the vein of R-Type or Gradius. The goal is to direct the SDI satellite and shoot down as many enemy units as possible as they fly across the screen. At the end of the process, the SDI satellite is collected by a shuttle and brought into low Earth orbit for stage 2: Defensive Mode.
In Defensive mode, the tally of fallen enemy units is shown and any remaining missiles are set on a course for Earth. From the low orbit perspective, the SDI satellite must destroy as many of the remaining missiles as possible. Failure to shoot down a sufficient number, either in Offensive or Defensive mode, causes a game over. This mode far more closely resembles that of Missile Command.