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In Pacific Rim series, Kaiju refers to the Precursors' series of titanic, drift-controlled monsters from Anteverse.
First Kaijus were developed by the Precursors sometime prior to or during the Triassic Period, yet at that time Earth's environment was too hostile for either them or Kaijus to enter, resulting in them halting their terraforming plan.
Thanks to pollution of Earth's environment by mankind, Precursors eventually able to continue their plan to take over the planet, all while wiping out all original lifeforms it. Creating the Breach, special portal which only beings with Kaiju DNA that can come across on the floor of the Mariana Trench, they deployed the first of Kaiju series, Trespasser, to wreak havoc upon human civilization starting San Francisco Bay and testing and weaken their best defenses. It took three tactical nuclear missiles deployed by the United States and British military to kill the beast when their conventional weapons and troops failed. The Trespasser incident at San Francisco was later known as "K-DAY".
Six months later, however, Hundun, Trespasser's successor, appeared and followed by others. With increased kaiju attacks and nuclear weapons proved ineffective, humanity formed PPDC and built giant mechs called Jaegers to combat the threat more effectively.
Over the next five years, kaijus begin to adapt with Jaegers' weaponry and begin to gain the upper hand. Notably, Knifehead managed to outsmart Gipsy Danger and killed one of its pilots, Yancy Beckett, leaving his brother and co-pilot traumatized. The construction of the Kaiju wall was commissioned when people begin to lose faith on PPDC, yet Mutavore's ability to tear it like a piece of glass and its subsequent defeat at hands of Striker Eureka proved it to be a fruitless endeavor. Hermann Gottlieb decided that Double Event, the moment where two kaijus appear at the same time will occur afterwards. While he was later proven right, he and the rest of PPDC soon learned kaijus' true nature as both combat beasts and terraforming units of Precursors thanks to Newton Geiszler drifting into one of captured kaiju brains.
Unfortunately, this revelation comes with a cost. As noted by Hannibal Chau, Geiszler's drifting into a Kaiju brain also allowed Precursors' drift system to locate him and scanned knowledge and secrets of PPDC that he had, leading them reworking on latest kaiju designs, Leatherback and Otachi to match the strength Cherno Alpha, Crimson Typhoon and Striker Eureka. In addition of overpowering three Jaegers, Precursors had Otachi tracking Geiszler down when the tide goes to their favor. Even so, the newly restored Jaeger Gipsy Danger appeared and defeated both kaijus much to their fury. To add insult to injury, not only Otachi's still developing fetus failed to kill Geiszler, but also he and Hermann drifted together into the baby Kaiju's brain, allowing PPDC to understand more of the Breach's system.
With another of their secrets revealed, Precursors initiated Triple Event by creating Raiju and Scunner both to guard the Breach and assisting their ultimate kaiju Slattern. Shortly after stealthily circling around Striker Eureka and Gipsy Danger to ensure whether there are opposition beside both Jaegers, Raiju returned to Scunner's side in time for Slatter's arrival. An intense battle ensued with Gipsy Danger lost its right arm and had one of its leg damaged from Scunner and Raiju's combined assault despite it managed to kill the former whereas Striker Eureka and Slattern deliver crippling blows upon one another, resulting the former's nuclear payload to destroy the Breach to stuck on its back. As a result, Striker Eureka had to blow itself up to kill both Slattern and Scunner when the two realized the Jaeger to be a greater threat, subsequently leading Raleigh and Mako, two pilots of Gipsy Dangers, to sacrifice their own since Gipsy is powered by nuclear reactor that can be used as a replacement bomb. After Striker Eureka's sacrifice, Gipsy Danger carries Raiju's remains as a shield to bypass Breach's scanner and enter Precursor's base in Anteverse, only to use Slatter's crippled form instead when the kaiju revealed to survive the point-blank blast albeit injured as result. But Slattern won't give up without a fight: It managed to cut off Mako's air supply as Raleigh used Gipsy Danger's turbine to fry Slattern's insides as they pass through the Breach, killing it and allowed the Jaeger to bypass the portal's scanners. Raleigh then ejected Mako in an escape pod while wasting no time in activating Gipsy Danger's self-destruct system before entering his own. As both pilots return to Earth, Gipsy Danger explodes, decimating the bewildered Precursors' facility and ending the war.