In the past the leader of the Elodians, Queen Ymir, did a deal with the devil to get the titan power. She used this power to conquer, she passed on her powers to her 7 followers so there are only 8 intelligent titans (Royal, Attack, Female, Armoured, Collossal, Beast, Jaw, Mule), the rest are mindless. At some point a country called Marley defeated the Elodians by somehow turning some of the titan shifters against them, half stayed and half fled to Paradis Island. This is probably not wholly true and is likely Marley propaganda.
Marley enslaved the ones who stayed and it's reminisent of 30s ghettos complete with Elodians having special armbands. These Elodians are ashamed of their people being the only in the world who can turn into monsters so let themselves be oppressed. The Marley use these brainwashed Elodian children to take the titan powers and win their wars. The Elodians who fled built the walls and had their memories wiped by the Royal Titan - the only shifter they had left. The enslaved Elodians hate these island Elodians because they abandoned them and could turn the walls back into 1000s of titans and destroy the world.
Erin's dad is part of a rebel movement and marries a women related to the royal family, their child is Zeke who they raise to be a rebel leader. He turns them in to Marley, so they are taken to Paradis island and injected to turn them into mindless titans to use against the islanders - all those random titans outside the walls are just Elodian rebels and criminals etc. Zeke is saved by one of the titan shifters who is secretly a rebel, shifters only live for 7 years after injection and his time is up, so he gives his Attack Titan power to Erin's dad. Zeke's mother is not saved and is actually the titan who killed Erin's mother, this is why Erin can use the Royal Titan's powers when he touched her in the last anime episode - she has royal blood. Erin's dad infiltrates the walls and eventually steals the Royal Titan power.
6 shifters (Annie, Bertholt, Reiner, guy Ymir ate, Zeke and Zeke's mount) are sent to get the Attack Titan power back and potentially the Royal Titan power and take the island, mainly they want to take the island because of it's oil reserves though. This is the original story told in the anime.
There is then a timeskip to about a year before Zeke's 7 year term as Beast Titan is over. Zeke and Adult Reiner are looking for their successors and the story is told from their point of view. Ymir has already been killed and her power passed on to someone else who uses it much better. Marley has a shitty military because they have relied on titans, but technology has progressed to the point where anti armour shells can potentially kill the Armoured Titan. They just about defeat the Middle East Union in a war with 1930s technology but the future isn't looking good with their conventional military so far behind. They have sent lots of ships to the island over the timeskip years but none come back. Reiner is having doubts about limiting his cousin to 7 years of life and has PTSD from when Mikasa fucked him up in the Shinganshima battle, Zeke has PTSD from when Levi fucked him up in the same battle.
It isn't clear yet if we will continue following the antagonists, but the story is making it very sympathetic for them. I don't know if they are going to invade the island again but I'm hoping that is island invades Marley as even though the scouts were destroyed, they still have the Military Police and the anti human ODM gear.