In the year 2015, the Observers invaded from the future, instituting "The Purge" and killing many humans. Although humans continued to resist well into the year 2036, the Observers largely succeeded in conquering the planet. [Source]
The Observers traveled back in time from the year 2609, where Earth had become almost uninhabitable. But why 2015? It seems odd to choose a time where humans are technologically fairly advanced, and therefore able to fight back (e.g. develop guns they can't dodge).
One explanation that comes to mind is that the Observers need certain resources that aren't available in earlier times.
- Why did the Observers chose the year 2015 for their invasion?