Oppenheimer participated in Project Charles during 1951, which examined the possibility of creating an effective air defense of the United States against atomic attack, and in the follow-on Project East River in 1952, which, with Oppenheimer's input, recommended building a warning system that would provide one-hour notice of an impending atomic attack against American cities. Those two projects led to Project Lincoln in 1952, a large effort on which Oppenheimer was one of the senior scientists. Undertaken at the MIT Lincoln Laboratory, which had recently been founded to study issues of air defense, this in turn led to the Lincoln Summer Study Group, in which Oppenheimer became a key figure. Oppenheimer's and other scientists' urging that resources be allocated to air defense in preference to large retaliatory strike capabilities brought an immediate response of objection from the United States Air Force (USAF), and debate ensued about whether Oppenheimer and allied scientists, or the Air Force, was embracing an inflexible " Maginot Line " philosophy. In any case, the Summer Study Group's work eventually led to the building of the Distant Early Warning Line.