Once Sundlun entered Switzerland he turned himself in to Swiss authorities and was interned along with other Allied military officers at a hotel in Davos. Within a few months, he was recruited by Allen Dulles to work out of the U.S. Embassy in Bern under the auspices of the Office of Strategic Services in the early preparations for Operation Sunrise. Later under the orders of Dulles, he reentered France to act as a bombardment spotter for the Allied invasion of Marseille in mid-August 1944. After a brief service as a pilot of C-54 Skymaster cargo planes into Karachi, and over "The Hump" to Kunming after VE Day, he ferried bombers (B-24 Liberators and B-29 Superfortresses) from the U.S. mainland to Tinian in the Mariana Islands and into other bases in the Pacific Theater of Operations.