In which chapter did the 6th movie of Broken Blade (Break Blade) end? Is it necessary to read the manga from the start, or did the anime stay faithful to it?
Short answer: the anime differs heavily from the manga starting from the 5th movie with the scene surrounding Rygart’s village. So, I’d recommend reading from chapter 27 (it starts with Girge’s past).
The producers of the anime likely wanted to end that arc with 6 movies. However, to be faithful to the manga you would need at least 1 more movie which is why they compressed and bent the story accordingly. So, you will find some scenes in the manga that seem similar to the anime, but they play out mostly completely differently and under a different context. This is just my opinion, but starting from that diversion, the anime became a mess, especially with what they did with Rygart’s fight against Girge that makes you want to punch our protagonist.
The first 4 movies however cover the manga very closely. So, you don’t need to read the manga from the start if you’re fine with missing some details.
There are a few differences I’d like to point out, though:
Sigyn’s action of why she takes in Cleo (after the latter became a captive) is better explained in the manga (read chapter 16 if you’re interested).
Remember that scene where Sigyn tells Cleo the story of Rygart and Hodr trying to sneak into the girls’ dormitories? In the manga, she confesses to Cleo in a roundabout way that she loves Rygart (see chapter 28).
Rygart’s a bit more level-headed in the manga (although not much more) and it's more apparent that he has a hard time trying to deal with his naive action of saving an enemy that ended with an ally soldier being killed. His „stupid“ (or call it ‚naive‘) actions are a bit more reasonable and there are also fewer of them, e.g. when Rygart’s been asked about the newly captivated Cleo, he doesn’t say to just let her free but talks about her overdeveloped body (see ch. 16).
Remember Nike, that crazy girl? If you found her sadistic side annoying, you will probably accept it more in the manga as it’s less shown and not so extreme.