2300 AD is a roleplaying game originated by GDW and currently owned by Tantalus Games. It uses the same timeline as an earlier game, Twilight: 2000, although the game systems were always different. Plainly speaking, the system was a ripoff of an obscure but quite interesting RPG called FTL 2448. GDW planned to convert 2300 to its "house" system like all its other roleplaying games, but went out of business before the revision was published.
To add to the confusion, the first edition of the game was called Traveller: 2300, although it used both different game mechanics and a different background from "classic" Traveller. GDW seemed determined to shoot itself in the foot on this one, and 2300 never did sell well enough to justify its existance from a business standpoint.
But 2300 was a pretty good game and the background for it, especially taken together with Twilight: 2000's, is a surprisingly rich one. There's no reason why it shouldn't be adapted to GURPS or another generic RPG system so that a new generation of players can discover and enjoy this worldline.
2300 (sic; I do know that the proper usage would be "AD 2300," but that's the official name of the game) gets its name from the arbitrary year set for the beginning of the campaign, three centuries after the world war detailed in Twilight: 2000. To understand the background, let's start with a brief summary of the "history."
This falls in the category of "alternative history" now (thank goodness.) In this world, Gorbechev died in the 1980s and the Soviet Union did not fall apart at the beginning of the last decade. Instead, the Soviet Union drifted into a war with China. This war went badly, and an attempt at German reunification lead to war in the West as well. In 1997, the war went nuclear, not a massive exchange but a gradual escalation. The U.S. government was devastated by a strike on Washington that took out both the President and the Vice-President.
The war sputtered on until the spring of 2000 when the last NATO offensive in Europe was crushed by a Soviet counterattack. Remaining US forces were evacuated the following year; the war was effectively over. By this time the typical military unit in Europe and the Middle East was primarily interested in maintaining itself, and feudalism returned for awhile. While the big war was over, there were many little wars to fight to settle small but important issues such as who gets enough food to live through the next winter. It was common for forces to make up their losses through local recruiting, and not unheard of for prisoners-of-war to be enlisted.
Meanwhile, the US enjoyed the services of no less than three governments: Civgov, headed by President Broward, whose election was not recognized by the Joint Chiefs, which became Milgov. Meanwhile a right-wing movement called New America took power in enclaves in the South, the Rocky Mountains, and even parts of the Alleghenies for awhile. The US was so weakened it took more than twenty years to suppress the last of the New American enclaves, and could not expel Mexican forces which had occupied Texas and about half the Southwest. About half the population of the United States was killed by the war or the effects of its aftermath.
France and Japan came out of the Twilight War with less damage; neither fought, although both suffered some bombardment (mostly oil refining facilities, in order to deny them to the other side.) France reestablished some of its colonies, and took effective control of the oil-producing regions along the Persian Gulf. France enjoyed pre-eminence for the next three centuries, though her star was finally setting again by the end of the 23rd century as other nations grew in relative power. "Presently" France is an Empire, with Nicolas Ruffin reigning. Interestingly, while Ruffin is thoroughly French in culture, he's of clearly African ancestry. In fact, the majority of French citizens live outside of Europe. France has built up a lot of resentment and would be in more trouble with the rest of Earth if it weren't for the war with the Kafers. Even with this, there is suspicion that France is trying to use the war to advance its own ends and accusations that French leadership has been less-than-effective.
Japan is a considerable power on Earth and has a population almost as large as the United States (although it isn't clear from the original sources whether or not this includes the population of the Philippines.) Japan has several interstellar colonies as well. Japan's space navy is not rated among the six greatest, but has done rather well in recent fighting with the Kafers, and a buildup of the fleet seems likely in the future, especially considering the comparatively poor performance of the French navy. Japan had chosen to align itself with France against Manchuria in its most recent war before the present one.
The United States has recovered and is definitely one of the greatest powers but it isn't the superpower any more and doesn't think of itself that way. US space ventures are almost invariably in partnership with Australia. Puerto Rico has become a state, and there is one extraterrestrial state, Ellis. On the other hand, Arizona has disappeared, the remainder of the state being united with New Mexico. Interestingly, the US stubbornly kept 50 stars on its flag even after losing Texas.
After being reabsorbed by Mexico, Texas won independence near the end of the 21st century and remained an independent nation. Rather surprisingly, given three centuries of French resurgence, Quebec remains a part of 24th century Canada.
Mexico has retained its hold on about half the Southwest, including about a third of California (although there's an interesting alternative alternative history if you want Hollywood to return to the USA for your campaign.) Mexico has also absorbed the rest of Central America (although Columbia got the Panama canal and the ithsmus southeast of it) and some Caribbean islands including Cuba. Mexico still isn't quite one of the great powers and even ranks behind Brazil and Argentina in the Americas. If the USA got a good excuse for war, Mexico would probably lose the North again, although it is questionable whether the northerners would want to join the United States or form their own nation, like Texas. Fortunately for Mexico, wars between human powers are less likely until something permanent is done about the menace of the Kafers.
China was broken up by the Twilight War and eventually coalesced into three nations: Manchuria, Canton, and a remnant "China" between the Hwang He and Yangtze rivers. Manchuria is the most powerful of the three and the largest, including not only the traditional Manchurian provinces but the Area around Beijing, Inner Mongolia, Sinkiang, Tibet, and quite a large portion of the formerly Russian areas including Vladivostok. Manchuria is one of the leading powers in space as well. Canton is a lesser power but still dynamic. The remainder of old China has become a sleepy backwater.
German reunification was delayed until the 2290s, largely thanks to French efforts. In particular, France supported an independent Bavaria which founded several extraterrestrial colonies.
Brazil, Argentina, and Azania (South Africa in this timeline) became considerable powers, with Argentina fighting the first interstellar war (between human nations.)
Azania is what South Africa calls itself in the 2300 worldline. The white minority government was replaced here, too, and Azania has absorbed a number of other Bantu-speaking nations on the southern end of Africa. A notable exception is Mozambique, whose small population have become very wealthy from tantalum production. The map of the rest of Africa looks quite different; the artificial borders left behind by the colonial era have been largely replaced. There are some entirely new nations, like Biafra and Polisaria. Azania is considered one of the major powers and has several interstellar colonies.
The Ukraine has done quite well and has a bigger presence off Earth than Russia. One of the more inspiring leaders to emerge from the Kafer war is a Ukrainian admiral, Sergei Borodin.
The FTL drive used in the 2300 universe works by "tunneling" through space in a series of short jumps—it's pretty much the same drive Poul Anderson described in his Nikolas van Rijn and Dominic Flandry stories. In GURPS Space terms it is a warp drive. It doesn't work deep in gravity wells, and it has a range limit of a little less than eight light years when a "charge" it builds up must be dissipated in a gravity well. This effectively constricts interstellar movement, and the accidents of astrography mean that the stars near Earth are now mostly organized into three arms: the French Arm (the most heavily settled), the American Arm, and the Chinese Arm.
Any world that is attractive to settlement will generally have more than one colony. Tirane, a very earthlike planet orbiting Alpha Centauri, is almost as heavily populated as Earth; the rest of the colonies are less developed and include lots of unspoiled areas. Not every system has a habitable planet, of course, but some systems with no earthlike planets have colonies nevertheless, either mining colonies exploiting some rare resource like tantalum (essential for stutterwarp drives) or way-stations for ships travelling through to other systems. The old Bavarian colony at Hochbaden was the largest settlement of a hostile-environment world. It was destroyed in the Kafer war; there were very few survivors.
A few extraterrestrial colonies have become independent nations. Whether or not this will be the wave of the future remains to be seen.
As of 2003, humanity is aware of seven intelligent extraterrestrial races. Evidence of Ebers was discovered long before they were. The Ebers are larger than humans, and once had a starfaring civilization. They have survived on three known worlds. Ebers look rather comical, with incredibly long arms, eyestalks, small trunks, and short legs. It is an often fatal mistake to underestimate them. Because of their peculiar biology, Ebers undergo periodic personality changes, so dealing with them can be extremely frustrating. One of the quests of the 2300 worldline is discovering the secrets of the ancient Ebers; they apparently had better stardrives than are now available.
Sung are smaller than humans and can fly on their home world and other similar ones. They were spacefaring when discovered, but did not have stutterwarp.
The Xiang are native to another world in the Sung home system. A short war was fought by a human alliance to "free" the Xiang, whom the Sung had made their clients. Xiang are a bit more massive than humans, and are octopods, with three pairs of limbs used for walking and one pair used for manipulation. They were in their bronze age when they were discovered by the Sung, and show little desire to adopt more advanced technology.
The Pentapods are quite strange. Individually they are small, little more than one meter tall. Pentapod technology is biotechnology; their spaceships are alive! Pentapods prefer to stay in their own enclaves but they do send trading missions to humans and have marketed quite a number of items of biotech, including a living biofilter mask and a cure for a grain blight which devastated agriculture on Beta Canum Venaticorum shortly before the Kafer War. (The real nature of the Pentapods is extremely weird and something no human knows.)
The Kafers have been known for only a few years. They took over a scientific outpost at Arcturus and invaded Aurore system in the late 2290s only to appear to withdraw. They came back in 2301 and for the next terrifying year bid fair to fight their way to Earth itself. They are a very warlike race, actually requiring the stimulation of danger and conflict to function at optimum levels. They are very roughly humanoid, bipeds with two arms, but the resemblance is minimal. They are called "Kafers" because the largely German-speaking Legionnaires which first encountered them thought they looked like big bugs. They have a carapace to protect their backs, and elaborate mouth-parts featuring palps which they also use as fine manipulators (their hands have basically three thumbs and don't grip as efficiently as a human hand.) They are stronger than humans on the average. They are also complete hermaphrodites; both partners of a mating usually have young.
The Klaxun were discovered in 2300. It's very difficult to communicate with them because they live on a different time scale. A Klaxun looks more or less like a leafless tree; it takes close observation to see one move.
The race most recently discovered are the Ylii, who actually comprise a number of related species, each forming a caste in Ylii society. The Ylii are technologically advanced but not warlike (not yet, anyway) and the Kafers are threatening to enslave or exterminate the Ylii. Paradoxically, one thing that has preserved the Ylii long enough to be discovered by humanity is that the Kafers like a good fight and the Ylii have not given them one—the Kafers literally have not been able to stay interested in war with the Ylii because they are too easy to beat. The Ylii are approximately humanoid but their head is just a holder for their eyes and ears; the brain is located in the upper torso, as is the mouth.
Material technology has advanced far enough to make the construction of beanstalks possible, and two have been built, one at Earth and one at Beta Canum Venaticorum (though the latter was disabled during the fighting with the Kafers.) A beanstalk is essentially an elevator cable running to a large satellite in geosynchronous orbit. Beanstalks make interface travel cheaper, and as a bonus, they also are considerable sources of energy (which comes from tapping the rotational energy of the system—beanstalks minutely slow down the rotation of their planets as they tap into this great reserve, but it would take a very long time for the effects to become noticeable.) It takes about five days to get from the surface to geosynchronous orbit, so more "conventional" interface transport is still competitive. Beanstalks have actually been proposed as future technology since the 1970s. Besides conventional rockets and hybrid scramjets there is a third method used to get from surface to orbit: the electromagnetic catapult. These are very big linear accellerators ("mass drivers" in the old Traveller terminology.)
Maglev trains also exist (magnetic levitation) as you might expect since they use much of the same technology as electromagnetic catapults, but the most common form of high-speed rail transportation is the airfilm train. These suspend the cars on a very thin cushion of air. Apparently this is more cost-effective on any planet with a sufficient atmosphere. Maglev lines are usually built on airless or very-low-pressure worlds, but since few of them have population or industry large enough to justify such a large investment, maglev lines are fairly rare.
Artifical gravity, contragravity and reactionless thrusters are not part of the established 2300 scene. However, the performance of interface transport (that is, ground-to-orbit) is clearly beyond the capabilities of chemical rockets, marring the "hard science fiction" feel to nitpickers like myself. It might be better to bring in the reactionless thruster to retain all those groundbased starfighters and tramp freighters.
There are energy weapons (lasers and plasma guns) in 2300 but most man-portable weapons still use projectiles. The most popular weapons are firearms using binary propellants. By separating the oxidizer from the propellant, substantial weight and bulk can be saved; guns typically have two or three times as many rounds in a normal load as 20th century weapons. There aren't any stunners, sonic or otherwise.
While large laser cannon do exist (the French have built battleships around them and the Kafers like them) the premier weapon for space combat is the detonation laser, which uses a small nuclear warhead to pump an x-ray laser. They are, of course, one-shot weapons, but that one shot is very powerful. Thanks to stutterwarp, all they really require are dispensers and interface fire control systems; typically a detlaser is ejected from a dispenser and fired when the mothership has warped hundreds of kilometers away. The detonation laser has been proposed since the late 1970s, most notably by Edward Teller, father of the US H-bomb.
An official attempt was made to graft cyberpunk onto 2300 but it was less than convincing. Still, such cyberpunk standbys as neural interfaces and weapons built into bodies were made available in the Earth/Cybertech book, so it's officially there if you want to use it. I don't think it fits the 2300 mythos very well, though.
There are mechas, or almost-mechas, in two forms: Human forces use thingies that are a little too big to be battlesuits but perhaps a little too small to be mechas. There's also a colony world where walking vehicles have become ridiculously popular for civilians.
Artifical intelligence doesn't appear to exist, and there is nothing like a "classic" robot in the established 2300 worldline.
Hovercraft are very popular in 2300 for both military and civilian use. Most of them can fly for short periods, trading off prodigious energy use for this capability. Once again, this is probably unrealistic, something that sneaked in because the designers were used to the ubiquitous "grav" vehicles of Traveller and wanted something that could do most of the same things.