From the lecture that David Gross gave to the Nobel committee in 2004:
Field theory was almost totally perturbative at that time; all nonperturbative techniques that had been tried in the 1950’s had failed. The path integral, developed by Feynman in the late 1940’s, which later proved so valuable for a nonperturbative formulation of quantum field theory as well as a tool for semiclassical expansions and numerical approximations, was almost completely forgotten. In a sense, the Feynman rules were too successful. They were an immensely useful, picturesque and intuitive way of performing perturbation theory. However, these alluring qualities also convinced many that all that was needed from field theory were these rules. They diverted attention from the non-perturbative dynamical issues facing field theory. In my first course on quantum field theory at Berkeley in 1965, I was taught that Field Theory = Feynman Rules. Today we know that there are many phenomena, especially confinement in QCD, that cannot be understood perturbatively.
Gross was awarded a Nobel prize in physics in 2004 for co-discovery of the concept of asymptotic freedom of the strong force. (Jointly awarded with David Politzer and Frank Wilzcek)