Innis's role as an artillery signaller gave him firsthand experience of life (and death) on the front lines as he participated in the successful Canadian attack on Vimy Ridge. Signallers, or spotters, watched where each artillery shell landed, then sent back aiming corrections so that the next shells could hit their targets more accurately. On July 7, 1917, Innis received a serious shrapnel wound in his right thigh that required eight months of hospital treatment in England.