Alternate History is historical fiction that examines the ramifications of a change or changes in known history. Alternate History can use science-fiction devices - such as time-travel - to change history, or it can simply pick a point where, had things merely happened differently (for example: Abraham Lincoln decided not to go to the theater that night, or Henry VIII stayed married to his first wife), history might have taken another path altogether. These books take place 20 years after the Worldwar series. The story begins with the arrival of the alien colonization fleet, and a new phase of the invasion brings thousands of extraterrestrials to Earth.