Schwinger's Quantum Action Principle. I. From Dirac's Formulation   through Feynman's Path Integrals to the Schwinger-Keldysh Method
  Starting from the earlier notions of stationary action principles, we show how Schwinger's Quantum Action Principle descended from Dirac's formulation, which independently led Feynman to his path-integral formulation of quantum mechanics. The connection between the two is brought out, and applications are discussed. The Keldysh-Schwinger time-cycle method of extracting matrix elements is described. Part II will discuss the variational formulation of quantum electrodynamics and the development of source theory. 
K. A. Milton
physics.hist-ph quant-ph