It was actually very simple. Shuizhou Country was surrounded by water, so to conquer Shuizhou Country, they had to cross the river first.
However, they were not good at fighting on the river. Once they fought, they would easily lose.
As for the Aduo Chai Kingdom and the Southern Border Kingdom, they were either on the prairie or in a land of ice and snow. They were also not suitable for fighting with the soldiers of the Black Cloud Kingdom.
Naturally, the Longyan Kingdom, which had a similar geographical environment and human resources as the Black Cloud Kingdom but was poorer and had a lower military strength, became a target in their eyes.
For the Dark Dragon Kingdom, if the Black Clouds Kingdom really succeeded in conquering the Longyan Kingdom, it would become an even more powerful country. Then, his next target would obviously be the Dark Dragon Kingdom.