Proof; Foo fighters: German-built weapons of war - first modern UFOs:

Wikipedia on «foo fighters»:

“The term foo fighter was used by Allied aircraft pilots in World War II to describe various UFOs or mysterious aerial phenomena seen in the skies over both the European and Pacific Theater of Operations.

Though "foo fighter" initially described a type of UFO reported and named by the U.S. 415th Night Fighter Squadron, the term was also commonly used to mean any UFO sighting from that period.

Formally reported from November 1944 onwards, witnesses often assumed that the foo fighters were secret weapons employed by the enemy, but they remained unidentified post-war and were reported by both Allied and Axis forces. Michael D. Swords writes:

During WWII, the foo fighter experiences of [Allied] pilots were taken very seriously. Accounts of these cases were presented to heavyweight scientists, such as David Griggs, Luis Alvarez and H.P. Robertson. The phenomenon was never explained. Most of the information about the issue has never been released by military intelligence.”

Regarding the last point, new information has come forward: It turns out that the U.S. government has known the truth all along. This is demonstrated in an official "Intelligence Digest" document, dated February 1945 which is made ​​public through FOIA:

The document deals with the German military capacity in 1945, and mentions "Phoo Bombs" as a weapon in the German arsenal.

"Foo Fighters" are in other words known as "Phoo Bombs" inside the intelligence.

It can thus be concluded that the "foo fighters" were German-built weapons of war, and that "foo fighters" represents the first modern UFOs.

This technology, and related technology in the form of "circular wing aircraft" (which can be proven was under development in Nazi Germany), was probably some of the technology one wanted to gain control of - through “operation paperclip” - and which may have been developed in U.S. black projects in the postwar years.