To answer your question, it did not amount to a full scale invasion in the zones that Shao Kahn occupied on Earth. Only surrounding citizens that populated the area were in danger before the menace spread around the world. It was more like, hundreds of thousands of souls were stolen and billions were threatened. Now, because of Liu Kang's triumph in MK2, in association with the tournament regulations, the elder gods allowed Raiden to temporarily defend the souls of the chosen ones who aided their champion, for different individual reasons. In the early events of MK3, the elder gods acknowledged the bigger plan and declined to intervene with Shao Kahn's tyranny, because they were not mortal. It was much later when the gods informed the elder gods that Kahn was attempting to merge the realms towards the end of MK3, that the elder gods took action to prevent this.