Background
The expansion of humanity into space has always been fraught with error and disaster, from the Challenger to massive cutbacks in funding delaying it even further, this trend continued well into the 21st century, a combination of politics, chance, and stupidity kept humans firmly on Earth until the discovery of certain new minerals and the possibilities they unlocked for all areas of technology. Before long, these materials were being mined and stockpiled by every nation, even though practical applications were only slowly being discovered. Conflicts over these resources provoked low level conflict virtually all across the globe, though full scale war was prevented for the time being. Then, with resource deposits nearly depleted and advanced technology finally coming into common use, eyes turned to space.
Diaspora
It was quickly concluded that TN minerals were only found on planets after Luna, Phobos, Deimos, and a portion of the asteroid belt were surveyed with no results, yet significant finds were made on Venus and Mars, settlement happened rapidly with multiple nations and corporate settlements bickering for space, fortunately there is quite a lot of that on empty planets so settlement, and later terraforming proceeded without serious incident, exploration of the rest of the system was not neglected either, despite the commonly accepted dogma of TN materials being only on planets, there was still a use for more traditional minerals like iron, or the rare materials used in high end electronics. With a small investment of Duranium, nearly self sufficient settlements could be set up for relatively low cost. This period became known as the Diaspora, as more and more people sought their fortunes in space.
There was an additional reason to leave Earth however, rampant industrialization had been causing increasingly severe climate effects for over a century by this time, and the advent of TN mining did nothing to change that, the side effects of unrestricted mining of exotic elements had added more radiation and new toxic chemicals to the environment, and Earth, while not lethal to human life, was undergoing a massive extinction event and becoming less and less comfortable.
All was not peaceful in space of course, human nature being what it is, political and religious infighting, combined with plain old fashioned cussedness, kept the military on edge and provided plenty of excitement, especially in the relatively unmonitored Kuiper belt, which was used as a convenient place for criminals, countries, and corporations alike to do whatever they like.
And then a discovery was made on Saturn's moon of Ganymede, a joint venture by American, European, and Japanese corporations discovered homeworld levels of minerals at easy accessibility on a moon, long though too small to permit the formation of TN materials. They acted quickly to secure the moon and to prevent the information from leaking, in this they were not successful.
War
As so often happens things degenerated from there, records from this time are not clear, it is known that survey of some of the other Jovian moons found minerals, but not in the levels that Ganymede had, it is also known that multiple attempts were made by several nations to annex the moon, but they were stood off by surprisingly advanced technology employed by the owning corporations. Which they proceeded to make available, for a fee, to any colony that wanted it, and many did. No one knows who actually fired the first shot, but for the first time in history, nuclear weapons were used against civilian targets, entire asteroid colonies seared from surfaces in a cycle of retaliation that ended up spilling over onto Earth itself, the various nuclear weapons on Earth had long since been converted to TN ICBMs, and the war was brief and final. Once the explosions stopped, the few irradiated survivors died in the nuclear ice age that followed.
The other planets were not spared in the fighting either, but once Earth went dark, the respective powers got a grip on themselves and managed a ceasefire.
Aftermath
That was 150 years ago, and humanity has spent the last century and a half in decline, getting a handle on their own worlds and becoming fully self supporting worlds. The small asteroid colonies have been absorbed or wiped out by the three remaining major powers.
Triumvirate of Venus The second world to be colonized, Venus was a multinational effort like Mars, but the increased difficulties of terraforming meant the residents had to spend more time in proximity and work together more to survive in the early days, which meant that when war came, infighting was at a minimum. The decision to have leaders from every major faction helped as well, currently there are three, hence the name Triumvirate. Venus, has perhaps the lowest amount of natural resources if the prewar surveys are to be believed, but rumors persist of a large convoy of freighters arriving from Earth in the days just before the war. The contents of which are a mystery.
Martian Union Primarily formed from the USA, but with assistance from the UK and France, the Martian Union formed officially during the chaos after the fall of Earth after conducting a rather brutal unification campaign on the surface of the planet, absorbing or... eliminating those who refused to toe the party line. Mars is in the position of having by far the most natural resources of any of the homeworlds.
Jovian Syndicate The corporate colony that controlled Ganymede managed to come through the conflict almost undamaged. With fair mineral supplies and a relatively intact command structure, they are well positioned in the post Earth world, despite having their own decline to deal with. The other nations still fear the possibility of advanced technology coming from the colony arguably responsible for the destruction of civilization as we know it.
Earth Nothing lives on Earth, nothing can live on Earth, a nuclear wasteland locked in a global winter, Earth's mineral deposits are long since mined out, but the various nations had massive stockpiles before the war, it is possible they still exist. Plus there may yet be installations of technology of worth for those willing to brave the radiation storms.
Other Ruined research colonies in the Kuiper belt or mining colonies throughout the system pose possibilities for exploitation, technological advancement was not even before the war and there may be other things out on the fringes of the system or even on bodies previously though fully exploited to loot, but there may be dangers as well, rumors persist of raiding vessels attacking the unwary, are they raiders from one of the established nations? Outlaw warlords? Rogue genetic abberations? Aliens? Or perhaps simply the fevered imaginations of crazed asteroid miners? Only time will tell.
