Before Mulder there was MacArthur, General Douglas MacArthur. The penultimate London Spectator quotes from a speech that MacArthur made to West Point cadets in 1955:

“The next war will be an interplanetary war. The nations of the earth must someday make a common front against attack by people from other planets.”

Obviously he had seen the wreckage of the Roswell spacecraft.

He knew.