Alien Alliance Invasion
In 2012, a highly advanced race known as the Dominators had taken notice of Earth and its burgeoning population of superheroes, particularly after the invasion by General Dru-Zod. The Dominators created an alliance to attack Earth before they could evolve into the next Kryptonian Hegemony, citing both the rise of new metahumans and the presence of many extraordinarily powerful aliens living on Earth. The Dominators and their long-time allies the Khund were joined by numerous species including the Thangarians, Daxamites, Badoon and Vegans. A vast joint fleet was sent to Earth.
The fleet arrived in mid-2012 and opened a major beachhead in Australia before Earth's defenders could react. The Fantastic Four coordinated an immediate counterattack, and quickly slowed down the advance of the invasion in the rest of the world. A detachment of Khund were repelled by an unidentified team of mutants in New York.
After less than two days of fierce fighting, primarily in Asia, and the virtual conquest of Australia, the Alliance offered a 24 hour cease-fire to allow Earth to consider their ultimatum: hand over all metahumans and aliens, or be destroyed.
To the surprise of the Dominators there seemed to be little appetite for their terms: humankind largely refused to surrender their metahuman allies. Many suspected the Alliance was using the ceasefire to construct teleportation gateways at their numerous small beach-heads around the world, as they had in Australia.
An hour before the deadline was up the superheroes of Earth concentrated their forces for a surprise counter-assault on Australia, a plan concocted by Batman, Superman, and Reed Richards. During this battle, Superman took on the Daxamites, and was briefly overpowered as the Daxamites were empowered by the radiation from Earth's sun. Victory seemed assured until the Daxamites flew over Beijing and were exposed to high levels of lead and other heavy metals. This weakened them to the point of death. Superman dragged them into orbit to save them. Shocked by this noble action, the Daxamites withdraw from the conflict.
Without Daxamite support, the invaders in Australia went on the defensive. Several smaller beachheads around the world suffered setbacks - SHIELD drove the Badoon out of Washington, a hastily assembled Chinese super-team routed the Thangarians in Jiangsu, and a Khund strike force in Metropolis was defeated by a group of young heroes. Four vigilantes on Manhattan Island spread out across New York to suppress opportunistic villains trying to exploit the chaos of war.
A group of heroes attacked Alliance ships and discovered that the Dominators had fabricated much of the evidence that they presented to the Alliance. Moreover they discovered plans to harvest genetic engineering data from humans and alter their genetics for military purposes via a Gene Bomb devised by Dominator scientists.
After seeing this evidence, the Daxamites declared for Earth and launched a surprise assault on the Khund fleet. The tide of battle started to turn.
Sensing the fight going against them, the Dominators attempted to withdraw. As a last-ditch measure the Dominators detonated their Gene Bomb. Though it caused little physical harm, the population of metahumans skyrocketed exponentially in the following months and years, a phenomena that is still being studied by Earth scientists.
In the aftermath of the invasion, several of the most prominent heroes involved in the defense of the Earth formed the Justice League. Other alien races aware of the attack begin to view Earth's defensive capabilities in a new light.