With the Best Picture at the Oscars this year going to Oppenheimer, I was reminded of this letter from Dr. J. Robert Oppenheimer and Professor Albert Einstein from June 1947 to President Truman. It seems like it never reached the President (as shown by some comments on page 6), but you can see the “TOP SECRET” stamp on it at the top.
The letter goes into detail about how the U.S. could go about communicating with extraterrestrials, assuming they would “have a psychology similar to that of men.” Oppenheimer and Einstein discuss at length various scenarios of visiting other-worldly beings and how the United States might handle situations, ranging from friendly to hostile encounters.
By the way, in the next to last paragraph on page 5 is an interesting passage: “Lastly, we should consider the possibility that our atmospheric tests of late could have influenced the arrival of celestrial scrutiny. They could have been curious or even alarmed by such activity (and rightly so, for the Russians would make every effort to observe and record such tests)”.