From the judgment of the International Military Tribunal in Nuremberg [pdf], p. 194:
It was contended before the Tribunal that the annexation of Austria was justified by the strong desire expressed in many quarters for the union of Austria and Germany; that there were many matters in common between the two peoples that made this union desirable; and that in the result the object was achieved without bloodshed.
These matters, even if true, are really immaterial, for the facts plainly prove that the methods employed to achieve the object were those of an aggressor. The ultimate factor was the armed might of Germany ready to be used if any resistance was encountered.
Full credit for the idea goes to Andrei Illarionov, the libertarian economist and a senior fellow at the Cato Institute, who quoted extensively from the part of the judgment concerning the Anschluss. I’ve kept it short.