Why did the Observers choose the year 2015?

Question

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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.

My Question:

• Why did the Observers chose the year 2015 for their invasion?

Answer 1

The full Invader (i.e. the Observers who hatched the invasion plan) plan was first to send 12 Observers back in time, these 12 then (unknowingly) scout history to find suitable times to invade. These 12 were told they were just studying history, and were the Observers we saw all over the place in the first 4 seasons.

With the scouting complete, the Invaders then analyzed history and found likely targets for when to invade. They have the capability to predict futures, so they applied this ability to determine the best time to invade. They found this perfect time to invade in 2015, and so they launched their attack.

This is revealed in episode S05E11, The Boy Must Live, when Windmark requests authorization to travel back in time in the present to catch the Fringe time at a time and place he knows they were. He's not allowed, as they have rules about time traveling during the time of the invasion, as they fear doing so would disrupt their 99.9999% chance of succeeding in the invasion.

Answer 2

Because it was after the merge between the two universes! They for some reason needed the two universes to become one before they could come back. (Maybe some of them were in one universe and some in the other? Or maybe the merge “changes” the universe in some way again (Walter admits that their universe crossing has changed both universes) that allows them to travel through time in mass?) The Machine is needed for that somehow, so they needed Walter and Bell to build the machine (presumably for their cross-universe studies; remember that Nina identified the blueprints for The Machine as looking like Bell’s work) and then they needed Jones to implement his plan to merge the two.

It’s currently either 2011 or 2012 in the Fringeverse. Assume that once the merge occurs it takes a few years for the universe to “stabilize” or the conditions to become right for them to do their mass time travel and invade. So 2015.

Source: http://14kofginafpd.com/posts/television/fringe-4-19-letters-of-transit/