After Sanji defeated Sheepshead at Zou, Shee:pshead’s female partner strongly implied that what led them to Zou was the pursuit of the ‘samurai’.
In other words, the purpose of coming to Zou was the pursuit of the ‘samurai’, & we know from several instances(and a theory) that Momonosuke must be the one that Kaido pursues.
When Sheepshead cornered Nami, Momo & brook, he implied that Zou is Kaido’s turf.
When Kinemon was narrating his escape from his pursuers(Kaido & his underlings), he said that his destination was Zou.
If Zou had been Kaido’s turf before Kinemon set out for it, his ‘escape’ from Kaido’s goons wouldn’t amount to an ‘escape’ then. He would be endangering Momonosuke by heading there because he will still be in Kaido’s ‘territory’. It would be like jumping from Kaido’s left hand into his right hand. It would have been a precarious act for someone who seeks to escape from Kaido to keep venturing into his territories, unless Zou hadn’t been Kaido’s territory then.
Back at Punk Hazard, Caesar said that he is well ‘protected’ by Doflamigo & and Kaido.
However, when they reached Zou 9 days ago, Caesar was feeling troubled with being at Zou.
If Zou had been Kaido’s turf before Caesar’s defeat at Luffy’s hands, Caesar would have known about that & would have felt more enthusiastic at being at an ‘ally’s territory’ because it could give him the opportunity of being freed from the captivity of the Straw Hats.
After Caesar was defeated by Luffy, Jack was suggested to become greatly displeased with Caesar’s defeat.
Both Jack’s attempt of rescuing Doflamingo and his predictable reaction of displeasure with Caesar’s defeat indicate that their defeats represent an inconvenience to him. In this case, he wouldn’t hesitate to liberate Caesar from the captivity of the Straw Hats when the opportunity arises. Aware of this scenario, Caesar shouldn’t have felt displeased with being on Zou if he had known that it had become Kaido’s turf, unless he hadn’t known about that. And the only way he would not have known about that is if it had become Kaido’s turf on the day that he had been at Dressrosa.
When Wanda was narrating the disaster that struck her country, she said that it was done by Jack in less than two weeks ago.
This timeline matches with the theorized time of Zou’s conquest.
Besides, it wouldn’t make much sense for Jack to destroy a country that had already been subjugated a while ago by Kaido, his boss, unless it hadn’t been his boss’ island yet.
When the Swirly Hats had arrived at Zou 9 days ago, Chopper found people injured, & there was also a panel showing smoke rising. These are all indications of a recent disaster.
We saw how strong the mink tribe is, especially the guardians during their battle against Luffy & Jean Bart. And we also saw that the guardians were bandaged and one of them had his arm in a sling. Those are all signs of an engagement in a battle against an overwhelmingly stronger foe. And that could only be Jack.
If their tribe (the mink girl) were chased by Kaido’s goons(Sheepshead) and the guardians weren’t there to put a stop to that, that most likely means that they weren’t able to do so. The only thing that would prevent them from putting a stop to Sheepshead’s base activities of harassing their own tribe would be a defeat against Jack.
For these reasons I believe that Jack invaded Zou in search of the ‘samurai’ & devastated the country in the process most likely the day before the Straw Hats’ arrival at Zou.