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a decade before zodiac's birth, the entire solar system aw burning in the fire of war between three forces. The humans, the A.I legion and the beast empire.it all started with w.w 3. the world was divided into two teams. the U.S and the Chinese.
at first, it was a minor diplomatic war in the early years it escalated to trade war when Americans found out that Chinese may take off their whole market they started banning Chinese industries. this led to a series of banning and counter banning each other. and this was all during a pandemic situation when world was already facing an economic crisis. which led to further destruction of the global economy.so to save economy all countries started taking their sides.
but the Chinese made a big mistake, it was their unwritten expansion policy which lead them to some minor border dispute with few fo its neighbouring countries in both land and water. This gave the U.S a big opportunity to weaken the Chinese economy and the U.S hugged this opportunity with open arms. in a month the Chinese side faced a knocking blow from U.S side leading to complete crumbling of the Chinese economy.
but this was not the end. The crumble of the economy worked as a fuse to the Chinese govt. and they declared war against the U.S. at first Chinese had an advantage as they gave a surprise attack and had greater manpower. but u.s countered it with a higher tier of military weapons. Then came the Chinese counter and its ace, the genetically modified and combined animals (a.n-like in Doraemon: Nobita's Great Adventure in the South Seas and Godzilla ) with the intelligence of a human (but it was sealed )boasting high defence, enough to withstand 10 times the power of a nuclear blast....but it had a drawback, I couldn't attack from long ranges and controlling them was problematic because they were remotely controlled with brain wave technology which was like human soul entering an animal body(A.N- just imagine your soul was exchanged with an animal will you able to control it your animal body properly )
with this U.S was now on backfoot. to counter it had to reveal its mechanical legion which was a troop of a million mecha troop but it was, do or die moment for both sides so Chinese side opened the brain locks from their animals except command locks in the brain, even though they knew that they may lose control of them they still took the risk. so now those animals were not puppets anymore but still, they had to listen to commands. In a single day, half of the mecha legion was wiped out.
with no choice, left U.S had to unleash its A.I legion who which was in the last phase of development, testing on security grounds was left out. they were highly intelligent (double of human to be precise) so they could modify themselves and they could be produced millions of millions on a single day. so again both side came to a deadlock.