The Allied Control Commission had a large pool of ex-German liners available to them following the Armistice. Imperator was temporarily allocated by the Allied Control Commission to the United States on 27th April 1919, to be used to repatriate American servicemen from the battlefields of Europe. Having been stuck in the river mud for over four years, four weeks of dredging was needed to free her. Once the boilers were lit the engines were found to be in a reasonable condition, and she sailed for Brest.
On 4th May 1919 she officially became USS Imperator, a Navy Transport, and was formally commissioned at 1.00pm on 5th May 1919 as Transport No. 4080. Over the next few days she was refitted to carry 1,000 officers, 966 NCOs and 7,939 enlisted men, with a crew of 2,200 US Navy officers and men. Leviathan, which had already been converted to a troopship, arrived at Brest on 13th May and a number of experienced officers and crew were transferred to USS Imperator. Troops began boarding both liners for the journey home.
On Sunday 5th May 1935 Normandie was ready for sea. In the afternoon the engineers started up the engines: at 5.00pm she eased away from the quay, and with the assistance of harbour tugs moved out into the Atlantic. Speed trials were performed over a measured course at Les Glénans, off the south coast of Brittany, on Monday and Tuesday. This was followed by handling trials. Normandie reached 30·156, 32·125 and 31·925 knots over three consecutive runs, without some of the boilers being fired up.
Tests included 8 hours continuous sailing in the Atlantic, during which she reached an average speed of 30·894 knots. There were three days of manœuvering and emergency handling trials, and equipment testing. Fuel consumption was even lower than expected. As soon as the trials were completed, Normandie headed for Le Havre, arriving on 11th May, where Captain Pugnet docked the massive liner without needing tugs, which were only used to swing the liner round as she entered the harbour.