The Invasion was the Dark Angels invasion of Earth and the annihilation of most human and animal populations.
The Invasion began when the Dark Angels arrived at Earth on a large meteor that broke off dozens of smaller rocks from their destroyed planet, before crashing into Mexico. The alien creatures, immediately after arriving, began a slaughter of the unprepared human populace, quickly overwhelming them with their armoured shells and hypersensitive hearing. It is assumed that several pieces of the meteor broke off, as the creatures reached Millbrook within minutes of arriving on Earth, despite the biggest fragment landing in Mexico.
While there is not much information about the battle against the aliens, it is implied through Lee's newspapers that Earth's militaries were unable to fight back against the Dark Angels due to their armoured skin and were easily overwhelmed, suffering grievous casualties. It is unknown if the soldiers managed to kill a single alien, but after finding they cannot swim, the US National Guard began evacuating people from the continent to the islands, with limited success.
Roughly a year-and-a-half later, the aliens have apparently achieved total dominance over the planet and wiped out the majority of any creatures that make noise. Humanity is now reduced to a scattered few survivors, mainly small groups hiding out in isolated locations, such as the Abbott family farm or the Island colony. However, by this point, the Abbot family had discovered how to kill the Angels, and had begun broadcasting their high-pitched frequency weakness.