(Note: this article is not finished yet, but it will be soon)
The Sacred Crusader Invasion was an imortant event in Earth's future history.
It started in 2015, when portals suddenly appeared all over our planet Earth. Out came savage aliens, who started to attack all humans, bent on killing them for being "heathens". It soon turns out that the race calls themselves the "Sacred Crusaders", and that they think they're on a crusade while killing the humans.
They're also technologically more advanced than us at the time. New York is lost in one of the first battles, also ending in the destruction for two whole regiments of US soldiers. Soon Washington is put under siege, defended by thousands of US military. Sweden was lost in the first few attacks, and guerilla war is fought against the invaders.