A sentence: It was dictated by the expansion of the empire, which had reached a scale in which human displacement had indeed become irreversible.
When we use SCALE, we must use ON
eg They entertain on a large scale.
eg Here was corruption on a grand scale.
eg On a global scale, 77% of energy is created from fossil fuels.
eg We have to evaluate performance on a scale from 1 to 10.
eg How much does it read on the scale?
But does it rule apply in that sentence?