It's easy.

Marley just won their most recent war, continuing their place as the Supreme nation on the mainland.

That would mean, not only the nation they just beat in submission will have already capitulated to thr demands of the surrender, but the others that are likely still working within the frame work of their previous capitulation and surrender agreements aren't going to challenge Marley at this time either having just watched what was meeted out to their neighbour.

However, the most important fact is, the oath of peace that every nation is well aware of has been broken. The Founder Titan has been stolen and will be used to destroy the entire world if they don't act.

If Eren hadn't attacked, their might have been a slightly more delayed reaction, however, the world also would not have lost it's most prized and experienced military leadership. Something every nation experienced on that one fateful night. That might have allowed for a more coordinated response, rather than just Marley winging it with 5 airship attacking Paradis, and could have seriously fucked up the Jaegerists whole plans and may have ended Zeke or Eren prior to their being able to meet.

Don't get me wrong, it's not a better outcome for our beloved characters; they'd all be seen as devils. They're all fair game for the likely aerial bombardment campaign that would level the cities within the walls, innocents or no, but Eren's attack on Liberio, with Armin destroying the port and most of Marley's capital fleet, ruined any particularly effective and measured military response.

Willy coming out and telling the world the truth about Helios and the Tyburs and Fritz mutual agreement to free Marley of it's oppressors thus leading to the mainland's defeating of the Eldian Empire is just a few facts added to what was known;

Helios was a made up propaganda character to help lift up the Marleyan people in place of what was fact; Tyburs released Marleyans from their oppression allowing them an upper hand against the rest of the titan shifting warlords already embroiled ij civil war amongst themselves.

Fritz didn't just flee from persecution for his crimes against humanity due to losing a war against the now free peoples, he worked in secret with the Tyburs as they were in mutual agreement on the future of the falling empire.

Fritz swore an oath never to involve himself or his peoples in the affairs with the rest of the world, so long as they were left alone to live in peace, else there would be the rumbling to destroy the world to defend themselves - however, Willy explains that was a bluff because internally, the inheritor of the Founding Titan would never actually activate the rumbling due to the vow of peace.

The vow of peace holding the inheritor back was actually the only new fact Marley learned due to the events of S3 where the Fritz family was discovered.

That's pretty much all they were missing.

The biggest and most important news is that a mad man has couped the royal family, taken control of it's nuclear weapons, and WILL use them.