Disclaimer: I don't own Animorphs, and you won't see them in this story. I do probably own any characters you don't recognize, though.
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A Brief History: Where Things Stand
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The Yeerk War was over. After the battle at Earth, the Yeerk Empire was gutted, and the Earth-Andalite Alliance emerged as the dominant force in the galaxy. Both worlds entered into new periods of prosperity, but massive changes rocked them as well.
For the Andalites, the new alliance signaled a huge change in policy. For decades they had lived by the law of Seerow's Kindness, using it and the chaos that ensued as a lesson to never interfere with the affairs of other races. With the nature of the victory at Earth, though, the Andalites basically had no choice but to open up formal relations with the planet. And even though the change was a huge one, many Andalite civilians welcomed it, having great respect for the humans after news of their victory reached the homeworld.
The changes on Earth were even more drastic, since they had opened up first ever relations with an advanced alien race (that wasn't trying to destroy them, at least). Z-Space and space travel marked the biggest breakthroughs with the technology transfer deals worked out between the humans and the Andalites, but other facets of life were altered, also. Automobiles, aircraft, phones, television, computers, PlayStations, and even toaster ovens all benefited from an infusion of Andalite technology. Politics were even affected, with some nations seeing the Andalites one-world federal government as the way to go. Borders changed and new world powers emerged as "Andalite democracy" became popular. Thailand, Cambodia, Vietnam, Malaysia, and Myanmar united into the Protectorate of Southeast Asia or the PSA and became the one of the dominant forces on the continent. The European Union went from a tight economic alliance to a nation united under one federal government. Persia was reborn when most of the Middle East united into one nation. All over the world, new unions were made. By 2050, only a half-century since the War's end, the number of nations in the world had been cut in half and the world scene was peacefully led by the United States, the PSA, China, and the European Union.
But, as mentioned, the biggest changes were in space travel. In 2006, a mere four years after the Andalites started sharing their secret, the United States launched the Jake Berenson. By Andalite standards it was nothing much, but it was Earth's first Z-Space craft, and its voyage from Earth to the Andalite homeworld was met with much fanfare.
From there the changes only sped up. In 2008, Earth's first private civilian spacecraft was launched by a firm in Japan. Two years later a Z-Space engine was added to it. By 2013 artificial gravity systems were perfected. In 2014 the USA landed on Mars, followed by the Europeans in 2015 and the Chinese in 2018. Starting in the early 2020s, settlements were established in orbit, on the moon, and on Mars. By 2030, terraforming of Mars began, followed by the colonization of several habitable, but uninhabited, worlds across the sector. By the 2060s all of Earth's space-faring nations had large fleets and armadas prowling the cosmos. By the end of the 21st Century the Andalites were starting to get a little worried; they had never expected a race could advance so quickly. Human civilization spanned across a half-dozen worlds and they all had powerful space forces. Andalite ambassadors to the United Nations, USA, PSA, China, and the EU voiced their people's disapproval to the human expansion, and tensions were raised. It was only a matter of time before things broke down altogether, and a massive human scientific breakthrough might prove to be the straw that breaks the camel's back…