The space aliens visiting the earth, who are really time travelers from the future, are the descendants of people that took part in space colony project and went out of the earth. The true purpose of the project is population reduction.
This population reduction project was carried out in 1999 in the original history. But they poured electromagnetic wave over the earth to try to the people left on the earth. That caused pole shift and nuclear winter. Both of these made the earth an ice ball. The earth had died. They were supposed to go back to the earth after killing all the people left behind but they wasn't able to.
The descendants succeeded in inventing time machine. As a result, they could go back from the future to the past. Their bodies had changed into aliens ones but they wanted to live on the planet on which their ancestors lived. They wanted to get the earth. The descendants of humans went back to the past living earth.
The group that had come back first was satanic group. They were satanic time travelers from the future. They stopped the space colony population reduction project and invaded the earth by deceiving the earthmen.
In order to do that, they used "plague and famine" and “microchip system”. They reduced the population and weaken the earthmen through plague and famine and economically controlled the earthmen with microchips system, which appears as beast number 666 in the Bible.