The legend of the sword says that it can destroy entire countries even against the user's will.
On the second attempt, he finally succeeded in taming it because he understood Kikoku's will and didn't treat it like some wild animal
Destroying the countries doesn't mean country level destructions. It’s a cursed blade, so that line isn’t literal. In One Piece, cursed swords like the Kitetsu line don’t nuke countries, they “destroy” through fate. Their curse drives wielders to madness or death, causing chaos and bloodshed wherever they go. So the sword’s legend means it brings destruction through its users, not by sheer power.
For example, Law plan to killing Doflamingo led to destruction of something huge, like beast pirates and Bigmom pirates.