The end of World War II on September 2, 1945, began a new era in many ways. Global attention turned from the destruction of the enemy to the rebuilding of countries and society. Massive reconstruction efforts began in Europe and Asia. In the United States, the movement toward the removal of segregation in the military initiated steps toward civilian desegregation. Justice was imposed on those who committed crimes against humanity. Simultaneously, a cold war began threatening the world with increasingly more powerful weapons of mass destruction. But the decade of 1945–1955 saw physicists turn tools created for war into medical instruments. High-powered microwave sources previously used for radar were coupled with newly invented corrugated traveling waveguides to create the first clinical linear accelerators. Reactors used to produce isotopes for weapons were used to generate brachytherapy and teletherapy sources. Cobalt-60 began its decades-long reign as the means of external beam irradiation. The new prevalence of radioisotopes brought about the need for radiation protection, detection instruments, and the profession of Health Physics.