Following information is based on a series of articles published in Sega Saturn Magazine before the release of Gungriffon II. Overseas version's narration of events will not be taken into account.
At the beginning of the 21st Century, majority of Earth's farmable land is lost due to conflicts and abnormal weather patterns, causing a worldwide food crisis. United States, suffering from economical problems, abolishes security treaties it had made with overseas countries and instead establishes "AFTA", an alliance of North and South American countries and enters an isolationist stance. Europe and CIS countries band together to form "PEU" while Japan, now without the protection of US security umbrella, forms "APC" with other Asian and Oceanic nations. Several African nations also come together to form "OAU".
PEU and APC are competing for influence in OAU's few developmental areas, due to the loss of American market and tensions flare soon enough. When Chinse-led APC forces in Libya launch a surprise attack on PEU forces in Egypt during a training excercise, World War III breaks out between these two superpowers. When China pressures Japan into sending forces overseas, Japan amends its constitution and forms Japanese Foreign Legion, an expeditonary force consisting of mostly foreign nationals to send overseas and prevent the deployment of Japanese Self-Defence Forces out of the country.
Main land weapons of the conflict are AWGSs, which stands for "Armored Walking Gun System", basically large manned robots with much better offensive capabilities and mobility compared to tanks and able to travel rough terrain. Trump card of the conflict is HIGH-MACS, an AWGS jointly developed jointly by Japan and US before the war. HIGH-MACS is capable of limited gliding and low altitude flight, giving it performance of not only a main battle tank but also limited capabilities of an attack helicopter.
Player is a member of the 501st (and later 504th) Mobile Anti-Tank Company "Griffon" of Japanese Foreign Legion, pilotting a HIGH-MACS.