The Invasion of Earth was a surprise attack, launched against the human race in general by an unknown alien species. It later appeared to have been masterminded and directed by the 'Overlords'.
The exact duration of the conflict is unknown.
The reasoning for the seemingly unprovoked and hostile invasion by the 'Overlords' is also unknown. However, their actions and activities following the close of the invasion closely resemble those of a military occupation, implying that the reason could be imperialistic.