While he was running for the Senate and carrying out his staff duties in London, the Invasion of Normandy and Operation Market Garden had taken place and Cain had missed both of them. He longed for an assignment in the field with a combat unit. In September, he was assigned to the XVIII Airborne Corps, commanded by Major General Matthew Ridgway, as Assistant Chief of Staff for Civil Affairs (G-5). During the Battle of the Bulge in December, Ridgway's Corps was instrumental in plugging the gap and Cain was in the thick of the fighting. Responsible for trying to protect and feed the civilians caught in the middle, Cain received a battlefield promotion to Colonel.