The Interplanetary Phenomenon Unit (or IPU) was a United States military unit established by at least 1947 and dissolved by the late 1950s.
Officials have confirmed that the IPU existed, but little else is known about it. It seems to have been an unidentified flying object-related undertaking. Some ufologists have suggested that the very name "Interplanetary Phenomenon Unit" is an indication that the IPU was convinced that the extraterrestrial hypothesis was a viable explanation for UFOs.
There has been speculation that the IPU was another name for the Majestic 12 or MJ-12, an unconfirmed (and controversial) U.S. UFO research group said to have been founded in 1947. However, another MJ-12 related document of questionable authenticity, indicates the unit was established early in 1942 by General George Marshall following a well-publicized UFO incident, the so-called "West coast air raid," in which an unidentified object over Los Angeles resulted in a massive anti-aircraft barrage. 
Others contend that General Douglas MacArthur was involved in the formation of the IPU, during or towards the end of World War II, because of the many UFO incidents occurring under his command in the Pacific. Allegedly MacArthur reported directly to General Marshall. Maybe supporting MacArthur's involvement is the fact that he did make public statements on at least two occasions that Earth might have to unite to fight a future war against an alien menace. (Two such quotes were in the New York Times, October 8, 1955, and July 5, 1961.)
In May 1984 researcher William Steinman first wrote the Army Directorate of Counterintelligence, since, according to Steinman's information, the IPU was run out of the Scientific and Technical Branch of the Directorate. Steinman received the following somewhat evasive reply from a Lieutenant Colonel Lance R. Cornine. Cornine claimed that the IPU had only an unofficial existence and refused to definitely acknowledge the existence of any unit records:
"As you note in your letter, the so-called Interplanetary Phenomenon Unit (IPU) was disestablished and, as far as we are aware, all records, if any, were transferred to the Air Force in the late 1950's. The 'unit' was formed as an in-house project purely as an interest item for the Assistant Chief of Staff for Intelligence. It was never a 'unit' in the military sense, nor was it ever formally organized or reportable, it had no investigative function, mission or authority, and may not even have had any formal records at all. It is only through institutional memory that any recollection exists of this unit. We are therefore unable to answer your questions as to the exact purpose of the unit, exactly when it was disestablished, or who was in command. This last would not apply in any case, as no one was in 'command'. We have no records or documentation of any kind on this unit." 
In March 1987, British UFO researcher Timothy Good also wrote the Army Directorate of Counterintelligence and again received a letter confirming the existence of the IPU from a Colonel William Guild. Guild was more definitive about the existence of IPU records and that they had been turned over to the U.S. Air Force Office of Special Investigations (AFOSI), the USAF counterintelligence unit, and the Air Force's Project Blue Book:
"...the aforementioned Army unit was disestablished during the late 1950's and never reactivated. All records pertaining to this unit were surrendered to the U.S. Air Force Office of Special Investigations in conjunction with operation BLUEBOOK." (letter in Good, Above Top Secret, p. 484).
Good also stated that the IPU reported directly to General Marshall. Documents from AFOSI about the IPU, if they exist, have never been released.