We see later in the series that the nation of Marley have been turning people into titans and dumping them on the shore of Paradis. But why? King Fritz warned the world that if the citizens of the wall were ever threatened by an outside force he would send the colossus titans inside the walls to flatten the Earth. So why send titans? I assume its a kind of revenge for the great titan war, but why risk the wrath of the king?
Actually the king didn't intend to use the rumbling, Tyler told this in the episode "Declaration of War" episode 4 of final season. So there wasn't this problem, they wanted the founding titan since the technological powers were growing, and hence the chances of losing to enemy countries. This was the reason they told too. Though it's not directly stated, but to me it may imply that they themselves intended to use the founding titans powers to create more controlled titans (Zeke's ability is limited) and threaten the neighbouring countries. Either way Marley has used titans this way only.– AdiG15Jan 26, 2021 at 11:39
To get the Founding Titan. I believe this was already explained in the manga and in the anime clearly.– W. AreJan 27, 2021 at 0:56
Is it not just a convenient way to get rid of troublesome Eldians without executing them. Titanize, no need to worry about anything after. They can't turn back/cause more trouble. The pure titans never threatened Paradis - they had walls and the king has the power to control. That's how I interpret it.– Shuri2060Jan 27, 2021 at 16:49
High-ranking people in Marley nation such as the Tybur family probably knew about the king's lie. As Willy Tybur admitted knowing this in the episode "Declaration of War" episode 4 of the final season.
Also, the beast titan (the predecessor of Zeke) knew about the oath. I don't remember the manga chapter.
However (perhaps) hiding this secret was used as a tactical strategy to motivate people to attack the island (not everyone is going to buy that " one gazillions years ago their ancestor killed your ancestor so turn them into zombies and lock them behind bars").
So because high-rank people knew about the bluff and hated Eldians for their "sins" and they wanted their resources and the founding titan, they keep harassing the island while used that bluff to motivate people against them at the same time.