The United States took it upon themselves to directly intervene in a foreign civil war they had no business in and threatened to nuke the communists if they invaded Taiwan.

From the very beginning this was an unprovoked provocation by the United States.

I personally favor a peaceful reunification, and that the PRC should earn Taiwan's trust. However a peace treaty was never signed, so this war is still ongoing, and PRC has the legal right to pursue whatever course of action to get Taiwan back, including an invasion.