The Advanced Aerospace Threat Identification Program (AATIP) was a Pentagon program to study unidentified flying objects. It officially ran from 2007 until 2012. The program was secret, but not classified. Its existence was first made public on December 16, 2017, at a press conference (where it was initially introduced as the Advanced Aviation Threat Identification Program). The press conference was co-organized by Luis Elizondo, who had been in charge of the program, and a related group of interested professionals from a nonprofit organization called, 'To the Stars Academy of Arts & Science.'
Note that in an interview in July 2018, Luis Elizondo said the name of the program has been misreported, and that its actual name was the Advanced Aerospace Threat Identification Program, and not Advanced Aviation Threat Identification Program.
The program was initiated by then U.S. Senator Harry Reid (D-Nevada) at the urging of Reid's friend, Nevada businessman and governmental contractor Robert Bigelow, and with support from the late Senators Ted Stevens (R-Alaska) and Daniel Inouye (D-Hawaii).
At first it was unclear whether this was a DIA (Defense Intelligence Agency) program, or not. The media said it was; the DIA initially denied, but it was later confirmed that this was indeed a DIA program.
The program ran for five years, with a budget of at least $22 million spread out over that time. (The NY Times was able to find contracts with subcontractors for a budget of 22 million USD over approx. 3 years).
The Advanced Aviation Threat Identification Program has generated a 490 page report that documents alleged worldwide UFO sightings over several decades. At present, that report has not been made public.
Given the few resources it had, it is likely that AATIP, like its predecessors - Sign, Blue Book, and Grudge - was more of a PR exercise than a genuine investigation. Like Col. Garret before him, back in the 40s, Luis Elizondo has gone on the record expressing his surprise at the complete lack of interest by his superiors with regard to his investigation.