Out of habit, whenever the outcome of a confrontation between powers became clear, other forces would at that time tacitly accept the results. Whichever side gained an advantage became an established fact, and no one would unduly interfere with that unless the next conflict arose. They would often allow those changes to solidify into reality.
It was exactly this mentality Zhou Tian exploited when he launched repeated attacks and quickly claimed a considerable expanse of territory on the Mainland. While it wasn't that he had become the undisputed ruler of the continent, Zhou Tian's gains there ensured that his expedition to the Mainland was a resounding success.
Zhou Tian's actions couldn't be deemed wrong, after all, he had indeed reaped immense benefits from constant expansion. Had he not expanded repeatedly on the Mainland, he likely wouldn't have owned his current foundation.