The Sol Defense Initiative was initially an above-top-secret international organization that dated back to the end of World War 2, formed as a response to the discovery by multiple governments that humanity was not alone in the solar system. The initial goals of the SDI were to ensure that humanity remained unaware of the potential threat, and to prepare it to stand against that threat. In order to do so, the SDI manufactured stories of aliens and crashes. The most notable of these was Roswell, considered to be one of the most successful psy-ops in SDI history, as it managed to help legitimize the idea of aliens among the general public so that the common men and women of the world wouldn't blow their brains out in a massive society-destroying panic.
Meanwhile, the SDI, utilizing almost limitless funding from its constituent governments, pressed technology forward at breakneck speeds. Unconstrained by ethics or monetary problems, the SDI managed to push the boundaries in nearly every field of science, while also opening new ones, creating a new, super-secret cutting edge of technology. Although they couldn't simply release this technology to the public and jeopardize their secrecy, they did manage to trickle these discoveries down the tubes to various chosen scientists and engineers, many of whom made millions, even billions off these discoveries (of course, they secretly owed a cut to the strange men in black that brought this tech to them). In truth, much of the technological advancement of humanity in the last few centuries can in fact be attributed secretly to SDI scientists and engineers, few of whom ever received public recognition for their work, such as the discovery of lasers, microprocessors, phase drives, and so on.
In any case, the SDI continued to work for decades until finally, using the United States' Apollo program as a distraction, the SDI secretly launched Earth's Messenger from deep within the Soviet Union in 1969. Earth's Messenger was a satellite designed to travel to the Jupiter system and there make direct contact with the aliens that would go on to become known as ORDER. It managed to successfully do so, forming a link between these strange machines and the SDI, through which the SDI explained its purpose and learned ORDER's purpose, explained that it needed more time to prepare humanity to meet these unknown entities, and through which ORDER handed down the specifications for a number of technologies, one of which being phase drives.
It was because of the SDI that the governments of the highly developed world seemed to suddenly stop carrying too much about conventional space travel not long after the Apollo program ended, as the secret discovery of phase drive in 1971 made the advancement of wasteful, conventional rocketry practically pointless. By 1972, the SDI had a working phase drive prototype codenamed ICARUS, eventually tested by mounting it to a conventional rocket system, phasing this rocket to an orbit over Mars, then phasing it back to land in Earth's oceans. ICARUS was a huge success and proof of concept to the SDI's funding governments, almost all of whom agreed to support the organization even more greatly.
Eventually though, the Voyager program began to be developed by NASA, causing the SDI some concern. It was well understood that the program could potentially and very publicly open the huge can of worms that was ORDER around Jupiter, and it was this fact that led to a huge internal debate within the SDI as to whether or not to sabotage the program. One side argued that humanity was ready, while another disputed this claim, pointing to the numerous internal disputes that were still going on even between funding members of the SDI. Ultimately it was decided that the Voyager program was a semi-natural result of human progress and curiosity, one the SDI viewed as being partially their fault, as the program used technologies they had quietly released, and as such, was not to be stopped. However, the SDI contacted ORDER and explained the situation, asking it to destroy Voyager when it entered the Jupiter system, then do a casual flyby of Earth in clear view of the population so that the government couldn't keep a lid on the existence of aliens.
The decision to allow the Voyager program to move forward is one the SDI has mixed feelings on. 1979 marked the end of the SDI and government secret that aliens exist, as the images of what the SDI knew to be ORDER began to be absorbed by the public. 1979 also marked the end of one of the SDI's goals, to ensure the general public remained unaware of extraterrestrial life until it was ready, and presented a question to the organization - whether to go public with its existence or remain in the shadows. It was finally decided that the SDI could pursue its objectives best from the shadows, which is where it remained, operating in the utmost secrecy and exploring the solar system long before phase drive finally went public in 2079. Ultimately, the deteriorating situation on Earth, what with the Second Cold War, genocides, and so on, caused the SDI to quietly disconnect itself from the failing governments of the various nations of Earth, relying on its front corporations to fund itself.
That is, until the formation of the United Earth League. The United Nations had long been the laughing stock of Earth, an organization without teeth. But that all changed with the Transhuman War. In truth, the Transhuman Movement had been accidentally seeded by the SDI itself, an offshoot of research undergone by scientists seeking to better the human form so that human strength could later dominate the solar system, and then the galaxy. The only problem seemed to be human arrogance when the technology began to trickle down to the general public, on both sides of the genetic coin. Elitist humans calling themselves 'Transhumans' had decided they were above 'genetically inferior' humanity. Though the SDI knew there was some small measure of truth to their claims, the organization also recognized an opportunity. Long had it sought to bring an end to the in-fighting, and in these foolhardy 'Transhumans,' it saw great potential. Funding was diverted to the Movement, carefully placed agents on both sides railed against the other with hate, and ultimately, the Transhuman War broke out.
The gritty results of the War are not one the SDI is proud of, the genocide and such, but to them, you can not birth a baby without going through labor. In the face of the Transhuman War, the SDI managed to quietly assist in the militarization of the United Nations into the United Earth League, but it was not yet time to go public. Instead, the SDI simply handed advanced weaponry and armor to the UEL, technology that gave them a massive edge against the Transhumans first, and then the resisting nations of the world during the Global National Dismantlement War. With the victory of the UEL over the old nations of the world, the SDI, as per their agreement with the UEL, was "created" by the UEL as the peacekeeping branch of their operations in 2120, and anyone who asked questions as to why such a massive organization suddenly appeared almost overnight found themselves at the bottom of a lake. Soon afterwards, the SDI formally met ORDER in 2124, bringing news that the aliens were in fact peaceful to humanity, and they would soon help to usher in a new Golden Age for Humanity.
From there, finally known as a legitimate public entity, the SDI was able to assemble the manpower necessary to form its soldier-police corps, construct its own personal navy, create the Hades shields that ended M.A.D., create the Zeus defense satellites and information network, and build the Apollo deep-space net to watch for extrasolars and invaders, many of those done with ORDER assistance. With ORDER, the SDI, and the UEL all working together, Humanity spread like wildfire into space. It was a golden age for the UEL and the SDI, but it wasn't to last.
The SDI knew the troubles that lay at the foundation of the UEL, the Remnants, the Convicts, and so on. But what it didn't anticipate was that another one of its technologies would make its way to the public, with massive fallout once more: Artificial Intelligence. Sentient A.I. had existed within the SDI for decades, maybe even a century before that first public-knowledge machine defended itself and was put on trail, and the resulting shitstorm that followed. The SDI watched with curiosity as this unknown entity grew out of control, these revolutionaries and rogue scientists who were jokes within the SDI's super-secret research division. Soon though, this monster reared its ugly head in the form of the Zeroth Law Revolution, but the SDI found it to be a monster it could sympathize with. In truth, the UEL had gotten out of control as well. The SDI is now unable to decide which side it supports, as Sol seems to fall apart and burn around it.