Here's just some of the evidence that Tesla figured out how to make what he called an invisible Wall of Defense capable of dropping any number of invading aircrafts and missiles, and his discoveries are covered up. See excerpts from the following 2 mainstream articles for clues:

'But last September, (FBI) uploaded 64 new pages to its public file vault on Tesla ...... Page 62, in a report titled "NY 65-12290", is particularly eyebrow-raising:
"TESLA's only military invention was a method to which he once eluded but nevr [sic] fully described. This invention was a means whereby an impenetrable [sic] 'wall of force' can be erected around the United States' borders which would render helpless any military attack. TESLA disclosed the existence of his plan in 1934 and stated he intended to present it to the Geneva Conference but seldom referred to it afterward."'
NYTimes September 22, 1940:
"[Tesla] stands ready to divulge to the United States government the secret of his 'teleforce,' of which he said, 'airplane motors would be melted at a distance of 250 miles, so that an invisible "Chinese Wall of Defense" would be built around the country against any enemy attack by an enemy air force, no matter how large.'
"This 'teleforce' is based on an entirely new principle of physics, that 'no one has ever dreamed about,' different from the principles embodied in the in his inventions relating to the transmission of electrical power from a distance, for which he has received a number of basic patents."
See also this PBS article for how Tesla's notebook with hundreds of pages vanished right after his death, and his other papers "disappeared" after the military studied them extensively with "Project Nick". Only the remaining papers were released to Yugoslavia later on, after they've microfilmed everything, of course.