The treaty was signed under threat of violence by all involved - that's what treaties after violence often do. Doh.

It preserved from republican rule a population that did not want republican rule. It would have been wholly wrong (at any time even up to now) to deliver those who did not want republican rule to the republicans.

At no time since 1922 has it been part of the English or northern Irish plan to change the government in the republic - whereas the republicans have at all times wished to and still do wish to change the government in northern Ireland.

That makes it quite clear who the invader is.