  We describe the earliest measurements of the DT fusion cross section
commissioned by the Manhattan Project, first at Purdue University in 1943 and
then at Los Alamos 1945-6 and later, in 1951-2. The Los Alamos measurements led
to the realization that a 3/2$^+$ resonance in the DT system enhances the
fusion cross section by a factor of one hundred at energies relevant to
applications. This was a transformational discovery, making the quest for
terrestrial fusion energy possible. The earliest measurements were reasonably
accurate given the technology of the time and the scarcity of tritium, and were
quickly improved to provide cross section data accurate to just a few percent.
We provide a previously-unappreciated insight: that DT fusion was first
reported in Ruhlig's 1938 University of Michigan experiment and likely
influenced Konopinski in 1942 to suggest its usefulness for thermonuclear
technologies. We report on preliminary work to repeat the 1938 measurement, and
our simulations of that experiment. We also present some work by Fermi, from
his 1945 Los Alamos lectures, showing that he used the S-factor concept about a
decade before it was introduced by nuclear astrophysicists.
