But divided what chance would they stand?
By the end of the second world war, over four dozen sovereign nations had entered into a temporary alliance against the axis powers.
It wasn't necessary for them to form a on-going federation in order to protect themselves from a common enemy. After the war ended, that alliance no longer served a purpose.
But, as NPM pointed out, without the existence of the federation the Empire might not have considered this area of any importance. Isolated Klingon ships might have raided from time to time, but no general attempt at conquest.