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How does choosing Imperials or Stormcloaks affect me later in the game?
Does joining either of these cause a split in the main story? Thus, consequently affect the quests available to my character?
I'm asking this because I aim to get all of the "complete quest x" based achievements in a single play through.
It does affect which quests are available, because joining one blocks your access to the other. However, it does not block your access to any achievements, because the civil war achievement is awarded upon completing either the Imperial questline or the Stormcloak questline. You do not have to complete both to get all achievements.
Pick whichever you want, and you'll still be able to get all achievements with one character.
This is just one story line in the game. Fighting off the dragons, which i belive is the main quest line isnt tied that tightly to the civil war, and hence will have no effect to it.
You can even get the savages to take over some of the places. :) Though This too isnt part of the civil war.
The game is immense in size, with loads of different quest lines to follow. Amazing game.
While it is possible to get all the main quest line and civil war quest achievements without any issues, you have to be aware of a possible conflict which may cause you to miss the 'War Hero' Achievement/Trophy.
According to the trophy guide here it is possible to miss the trophy if you haven't finished the civil war quest line before getting in too far into the main quest line. In the main quest line 'Season Unending' you have to negotiate a peace treaty between the 2 factions. During the course of these negotiations you have to make sure that the 2 forts (Sungard and Greenwall) either end up with different factions or they both end up with the faction you are not planning on being aligned with.
A bit confusing I know. And to add more frustration to this situation, according the uesp wiki the 'Season Unending' quest has a number of bugs. So if you want all the achievements for sure, I strongly recommend finishing the Civil war quest line before the main quest line. This way you never have to go though the peace treaty negotiations and will have already acquired the 'War Hero' achievement.
Side note: Also, take a look at the miss-able achievement 'Oblivion Walker' at the link above.
