Why was Kirigakure the least threatening after its destruction? They had two Tailed Beast Jinchuriki. If they truly cast aside all restraint and wreaked havoc, they might even drag the Shinobi World into the apocalypse.
This had to be analyzed from both Kirigakure's geography and politics.
First, Kirigakure was isolated overseas. This was both an advantage and a disadvantage, and everything depended on the strength of its navy. Because of this unique geographical location, after destroying Kirigakure, Nawaki could completely blockade the entire Land of Water with carrier groups. Without ships, Kirigakure's remaining forces would be unable to reach the mainland. At that point, they could at most wreak havoc in their own homeland.
Second, politically, Yagura's relentless high-pressure Blood Mist policies over the past decade had led to constant infighting within Kirigakure, while its atmosphere had grown increasingly rotten. Every ninja had become a cruel weapon of war. Although such tool-like ninjas were useful, it had also worn away their souls. The vast majority obeyed only to survive; in truth, they no longer felt much belonging toward the brutal Kirigakure. Let alone a conviction like the "Will of Fire." Once something happened to Yagura, who held the weapons in hand, these tool-like Mist Ninja would immediately collapse and ultimately become scrap metal.
In the original story, after Yagura was controlled by Obito, all of Kirigakure appeared normal on the surface, but in reality, it was like a vegetative patient who had long since lost consciousness. The entire political situation was in utter chaos. Kirigakure became synonymous with cruelty and bloodshed, with rebellions and vendettas playing out without end. Even when they eventually discovered that Yagura, the Mizukage, had been controlled by genjutsu, no one wanted to seek justice for him. That showed just how unpopular Yagura was in Kirigakure.