@MrBeaver
Eldia ( as far as we know, since it is canonical that the historical records have been tampered on both sides and the Titans genetic memory only goes so far ) was the empire that either invented or extensively abused the titans power to gain power over the world. Somehow ( that part is really not explained as much as that ) they were defeated and they were permitted to retreat to Paradis island and made a walled city with Titans. So, basically everyone in the cast is Eldian
Marley is one of the sucessor states that picked up from what Eldia left behind. They have a significant, but heavily discriminated, minority of Eldians ( probably left behind when Eldia retreated to Paradis island ) and it is somewhat infamous for picking the Eldian habit of using Titans ( most of them unwilling Eldian conscripts ) in warfare. It was Marley that decided to make a recon in force of Paradis island with 3 Titans a couple of years ago .... You might remember that
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That said, I have my mixed feelings about Eren actions. First of all, I have my big doubts about all the show he made with Misaka. Sure, he is the Coordinate and all of that, but if you think about it, if Eren could really compel conscient titans to do his bidding like he implies he can ( remember that he states that the Ackerman are forced to protect him because they are mini-titans and that the Coordinate power compels them to protect him ), then none of the attacks he suffered from Titans ( including the punches of Arwin in that same chapter ) would never have happened at all. It is like he was trying to push Misaka and Arwin away from him *cue march of the solitary hero*
And, let's be honest, even disregarding all of that, what the hell was the point of Eren going to Marley? OFC the stated reason was to make a big bruahh and force a attack on the Titans ( title drop
) Island, but it was not that Marley was not planning to attack Paradis Island already
. No, Eren went there for some other reason ... most likely intel gathering. And who knows what he discovered ...