He invented matrix mechanics[?], the first formalization of quantum mechanics in 1925. His Uncertainty Principle, discovered in 1927, states that the determination of both the position and momentum of a particle necessarily contains errors, the product of these being not less than a known constant. Together with Bohr, he would go on to formulate the Copenhagen interpretation of quantum mechanics.
Nuclear fission was discovered in Germany in 1938. Heisenberg remained in Germany during World War II, working under the Nazi regime. He led Germany's nuclear weapon program, but the extent of his cooperation has been a subject of controversy.
He revealed the program's existence to Bohr at a conference in Copenhagen in September 1941. After the meeting, the lifelong friendship between Bohr and Heisenberg ended abruptly. Bohr later joined the Manhattan Project. Germany did not succeed in producing an atomic bomb.
It has been speculated that Heisenberg had moral qualms and tried to slow down the project. Heisenberg himself attempted to paint this picture after the war, and Thomas Power's book "Heisenberg's War" and Michael Frayn[?]'s play "Copenhagen" adopted this interpretation.
In February 2002, a letter written by Bohr to Heisenberg in 1957 (but never sent) emerged. In it, Bohr relates that Heisenberg, in their 1941 conversation, did not express any moral problems with the bomb making project, that Heisenberg had spent the past two years working almost exclusively on it, and that he was convinced that the atomic bomb would eventually decide the war.
Most historians of science take this as evidence that the previous interpretation of Heisenberg's resistance was wrong, but some have argued that Bohr profoundly misunderstood Heisenberg's intentions at the 1941 meeting.
He wrote a book called "A Part and The Whole" about his life, his friendship with Bohr, and the evolution of quantum physics.