While studying d(d,n)3He fusion in 1938, Ruhlig observed protons with energies larger than 15 MeV. Ruhlig suggested that these high-energy protons were generated by tritium-on-deuterium fusion neutrons scattering protons out of a thin cellophane foil placed inside a cloud chamber. This led Ruhlig to hypothesize that he was observing secondary (in-flight) tritium-on-deuterium fusions and conclude that the d(t,n) reaction “must be an exceedingly probable one.” This was the first attempt to quantify the probability of d(t,n) fusion, using the ~1-MeV tritons generated by d(d,p)t fusion. This caused some Manhattan Project scientists to suggest that the d(t,n) cross sections are significantly higher than those for deuteron-on-deuterium fusion and led to the first measurement of d(3He,p) and d(t,n) cross sections in 1943. Here, we have used modern cross sections and stopping powers to estimate the expected numbers of high-energy protons associated with in-flight d(t,n) reactions in Ruhlig’s experiment. Our estimate is four orders of magnitude lower than Ruhlig’s observed rate. However, the number of high-energy protons in Ruhlig’s experiment can be obtained via simulation if the protons are assumed to have been emitted by secondary in-flight d(3He,p) reactions, with various plausible assumptions about the experimental geometry and target-backing thickness. Our calculations demonstrate that quantitative information about the fusion of A = 3 ions with deuterium could have been obtained via experiments similar to Ruhlig’s well in advance of the advent of 3He ion and triton beams in 1943. This opportunity seems to have been missed.