Aliens are observing Earth as a zoo apparently. They deliberately (or due to alien miscalculations) crashed their UFO's into Nazi Germany (1937), the United States (1947), and the Soviet Union (1986). They were probably interested in seeing how their technology would be used.
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They are not invading because unlike the 50's today everyone is with a phone in their hands taking pictures and videos of everything that moves. They're just shy, thats all.1