This question contains spoilers for Season 7 that can't be completely hidden by spoiler areas. So please stop reading if you haven't watched it.
After watching season 7, I was wondering - how could the Army of the Dead have invaded Westeros without the lucky fluke of a dragon flying into their territory and being killed?
If we know that the Wall's magic keeps them out, then how did they plan to attack Westeros, before they got a dragon?
Wouldn't the Wall's magic have prevented them from storming the gates in the Wall?
It seems very strange to me that the White Walkers had been gathering their army and preparing to invade for so long, without having the means to carry it out.
I thought up the following possibilities:
The White Walkers can, in some way, see the future. Perhaps their plan was built around some prophecy of this event.
They had some unknown item that were planning to use but was less effective than the dragon, so they used the dragon instead.
I'm wrong about the Wall's magic and they could have broken through the gate like Mance Rayder attempted to.
They weren't really going to invade and the invasion is mainly a device to speed up the plot, since there are only 6 episodes left. However, I'm doubtful that they'd go quite so far from G.R.R Martin's plan for the plot. (It's a pity that the most exciting part of the grand plot is being compressed so much.)
More thoughts about this matter would be very interesting.
It's ironic that the invasion seems to have begun because of Jon Snow's efforts to prepare for it...