Classicly

Xenthl had pulled a dirty one. Forty years ago, the people of Iaaro, system of Sirius, under the too-able leadership of Xenthl, had finally made their presence known on Earth. For three centuries, they had examined Earth from afar to assure themselves that it would make a suitable landing spot. For awhile, during the mid-twentieth century, they had scared the pants off some people and incurred the ridicule of some others less imaginative with their disc-shaped space and aircraft, but that couldn't have been helped. One had to trace a culture, a civilization, for a long time before one could analyze its true character-traits, abilities, potentialities. Especially when one's forces were few in number, and there were nearly two billion of the other fellow.