Karl Fritz could have brought the rumbling all over the world, but he always hated the war.
That's why he vowed not to have a war again (vow of renouncing war), also he vowed for whoever with royal blood has the Founding Titan will bound to this agreement that they will never wage a war in their lifetime. Also, the vow states that only people with royal blood can use the absolute powers of the Founding Titan.
Here's a piece of solid evidence, even if it's a spoiler, you can view this because it's not a major one:
And this has been always known by the rest of the world that the Fritz family will never wage a war against the world. This was the only reason that the Marleyans were never really afraid of attacking the Paradis Island, even though they were aware of the Founding Titan's power.
So basically, they just wanted the Founding Titan back (the Reiss family wasn't really using it) so that they can use the Founding Titan's power to gain control over other territories, because other territories were advancing in terms of weapons to take down the titans (like Beast and Armored). This can be confirmed from the starting episodes of the final season, where an armored train literally overpowered the Armored Titan, also later Armored Titan helps Beast Titan to destroy the ships, which first attacked Beast Titan (if Armored Titan wasn't there, Beast Titan would've fallen down.)
But later, when the Marleyans know the truth that the Founding Titan was stolen from the Reiss family (formerly known as the Fritz family), the Marleyans started to be afraid that the rumbling will happen sooner or later because the vow was not bounded to the person who doesn't belong to Fritz family but also, to start the rumbling, the Founding Titan holder needs royal blood. That's why they held the festival (which can be seen in the latest episode released on 28th Dec) so that they can regroup the other countries and attack on the Paradis Island before people from Paradis island start the rumbling.
Here's the major spoiler from the manga, which justifies the first statement:
When Zeke and Eren work together, Zeke wanted to command Ymir, so that the people from paradise island can never have a child. So Zeke needs his help as Eren cannot use Founding Titan's power unless he touches someone with royal blood. Zeke wants his cooperation to implement his plans. So, Zeke describes the vow to Eren. See how a person with royal blood is chained down so that vow can be maintained.