Nikola Tesla/Later years
Prior to the First World War, Tesla looked overseas for investors to fund his research. When the war started, Tesla lost funding he was receiving from his European patents. Wardenclyffe Tower was also demolished towards the end of WWI. Tesla had predicted the relevant issues of the post-World War I environment (a war which theoretically ended) in a printed article (December 20, 1914). Tesla believed that the League of Nations was not a remedy for the times and issues. In 1915, Tesla filed a lawsuit against Marconi attempting, unsuccessfully, to obtain a court injunction against the claims of Marconi. Around 1916, Tesla filed for bankruptcy because he owed so much in back taxes. He was living in poverty.
Tesla started to exhibit pronounced symptoms of obsessive-compulsive disorder in the years following. He became obsessed with the number three. He often felt compelled to walk around a block three times before entering a building, demanded a stack of three folded cloth napkins beside his plate at every meal, etc. The nature of OCD was little understood at the time and no treatments were available, so his symptoms were considered by some to be evidence of partial insanity and this probably hurt what was left of his reputation. This obsessive-compulsive behavior may have originated from the observations over repeated polyphase systems in nature that Tesla researched.
At this time, he was staying at the Waldorf-Astoria Hotel, renting in an arrangement for deferred payments. Eventually, the Wardenclyffe deed was turned over to George Boldt, proprietor of the Waldorf-Astoria to pay a $20,000 debt. In 1917, around the time that the Wardenclyffe Tower was demolished by Boldt to make the land a more viable real estate asset, Tesla received AIEE's highest honor, the Edison Medal. The irony of this honor was probably not lost on Tesla.
Tesla, in August 1917, first established principles regarding frequency and power level for the first primitive radar units in 1934. Emile Girardeau, working with the first French radar systems, stated he was building radar systems "conceived according to the principles stated by Tesla". By the twenties, Tesla was reportedly negotiating with the United Kingdom government under Prime Minister Chamberlain about a ray system. Tesla had also stated that efforts had been made to steal the "death ray" (though they had failed). The Chamberlain government was removed, though, before any final negotiations occurred. The incoming Baldwin government found no use for Tesla's suggestions and ended negotiations.
On Tesla's seventy-fifth birthday in 1931, Time magazine put him on its cover.  The cover caption noted his contribution to electrical power generation. In 1935, many of Marconi's patents relating to the radio were declared invalid by the United States Court of Claims. The Court of Claims decided that the prior work of Tesla (specifically US645576 and US649621) had anticipated Marconi's later works. Tesla got his last patent in 1928 on January 3, an apparatus for aerial transportation which was the first instance of VTOL aircraft. In 1934, Tesla wrote to consul Janković of his homeland. The letter contained the message of gratitude to Mihajlo Pupin who initiated a donation scheme by which American companies could support Tesla. Tesla refused the assistance, and chose to live by a modest pension received from Kingdom of Yugoslavia and to continue researching.
When he was eighty-one, Tesla stated he had completed a Dynamic Theory of Gravity. He stated that it was "worked out in all details" and hoped to give to the world the theory soon.  The theory was never published. At the time of his announcement, it was considered by the scientific establishment to exceed the bounds of reason. Some believe that Tesla never fully developed the Unified Field Theory, nor that any physicist in the years since it was first postulated.
While Tesla had "worked out a dynamic theory of gravity" that he soon hoped to give to the world, he died before he publicized any details. Few details were revealed by Tesla about his theory in the announcement. Tesla's critique in the announcement was the opening clash between him and modern experimental physics. Tesla may have viewed his principles in such a manner as to not be in conflict with other modern theories (besides Einstein's). Tesla's theory is ignored by some researchers (and mainly disregarded by physicists).
The bulk of the theory was developed between 1892 and 1894, during the period that he was conducting experiments for with high frequency and high potential electromagnetism and patenting devices for their utilization. It was completed, according to Tesla, by the end of the 1930s. Tesla's theory explained gravity using electrodynamics consisting of transverse waves (to a lesser extent) and longitudinal waves (for the majority). Reminiscent of Mach's principle, Tesla stated in 1925 that,
- There is no thing endowed with life - from man, who is enslaving the elements, to the nimblest creature - in all this world that does not sway in its turn. Whenever action is born from force, though it be infinitesimal, the cosmic balance is upset and the universal motion results.
Tesla, concerning Albert Einstein's relativity theory, stated that '...the relativity theory, by the way, is much older than its present proponents. It was advanced over 200 years ago by my illustrious countryman Boskovic, the great philosopher, who, not withstanding other and multifold obligations, wrote a thousand volumes of excellent literature on a vast variety of subjects. Boskovic dealt with relativity, including the so-called time-space continuum...', (1936 unpublished interview, quoted in Anderson, L, ed. Nikola Tesla: Lecture Before the New York Academy of Sciences: The Streams of Lenard and Roentgen and Novel Apparatus for Their Production, 6 April 1897, reconstructed 1994).
Tesla was critical of Einstein's relativity work, '...[a] magnificent mathematical garb which fascinates, dazzles and makes people blind to the underlying errors. The theory is like a beggar clothed in purple whom ignorant people take for a king...., its exponents are brilliant men but they are metaphysicists rather than scientists...', (New York Times, 11 July 1935, p23, c.8).
Tesla also stated that 'I hold that space cannot be curved, for the simple reason that it can have no properties. It might as well be said that God has properties. He has not, but only attributes and these are of our own making. Of properties we can only speak when dealing with matter filling the space. To say that in the presence of large bodies space becomes curved is equivalent to stating that something can act upon nothing. I, for one, refuse to subscribe to such a view.', (New York Herald Tribune, 11 September 1932)
Death and afterwards
Tesla died alone in the hotel New Yorker of heart failure, some time between the evening of January 5 and the morning of January 8, 1943. Despite selling his AC electricity patents, he was essentially destitute and died with significant debts.
At the time of his death, Tesla had been working on some form of teleforce weapon, or death ray, the secrets of which he had offered to the United States War Department on the morning of 5 January. It appears that his proposed death ray was related to his research into ball lightning and plasma. He was found dead three days later and, after the FBI was contacted by the War Department, his papers were declared to be top secret.
Immediately after Tesla's death became known, the FBI instructed the Office of Alien Property to take possession of his papers and property, despite his US citizenship. All of his personal effects were seized on the advice of presidential advisors. J. Edgar Hoover declared the case "most secret", because of the nature of Tesla's inventions and patents. Tesla's Serbian-Orthodox family and the Yugoslav embassy struggled with American authorities to gain these items after his death due to the potential significance of some of his research. Eventually, his nephew, Sava Kosanovich, got possession of some of his personal effects (which are now housed in the wikipedia:Nikola Tesla Museum in Belgrade, Yugoslavia). Tesla's funeral took place on January 12, 1943 at the Cathedral of Saint John the Divine in Manhattan, wikipedia:New York City.
In 1976, a large bronze statue of Tesla was erected at Niagara Falls State Park. A similar statue was also erected in the Tesla's hometown of Gospic in the 1981. The statue in Gospic was dynamited by the Croatian forces in 1991.
Perhaps because of Tesla's personal eccentricity and the dramatic nature of his demonstrations, conspiracy theories about applications of his work persist. The common Hollywood stereotype of the "mad scientist" mirrors Tesla's real-life persona, or at least a caricature of it—which may be no accident considering that many of the earliest such movies (including the first movie version of Mary Shelley's Frankenstein) were produced by Tesla's old rival, Thomas Edison. There are at least two films describing Tesla's life. In the first, arranged for TV, Tesla was portrayed by Rade Šerbedžija. In 1980, Orson Welles produced a Yugoslavian film named Tajna Nikole Tesle (The Secret of Nikola Tesla), in which Welles himself played the part of Tesla's patron, George Westinghouse.
According to FBI documents acquired via FOIA request, the sum of Tesla's possessions ("consisting of about two truckloads of material... [and] approximately thirty barrels and bundles") were seized, upon his death in 1943, by agents of the (now defunct) Office of Alien Property Custodian. One document states that "[he] is reported to have some 80 trunks in different places containing transcripts and plans having to do with his experiments..."