With America's entry into World War I in April, 1917 Hawes resigned from the Missouri House to serve in the military. Commissioned a Captain in the U.S. Army, Hawes served in the Psychological section of Military Intelligence. Working in France and Spain during the war, he was eventually assigned as military attaché to the U.S. Embassy in Madrid. Promoted to Major, Hawes was discharged in 1919. Returning home to Missouri, Harry B. Hawes was elected to the Missouri's 11th congressional district, United States House of Representatives in the election of 1920, edging out Republican Bernard P. Bogy by a little over 2,000 votes. Alleged voting irregularities, including destroyed ballots, led Bogy to mount a legal challenge to the election outcome. Hawes counter claimed that Bogy was not a legal resident of the 11th Congressional district thus ineligible to serve. Several weeks of legal maneuvers followed before Hawes was again certified as the election victor. He would subsequently be reelected in 1922 and 1924, serving in the Sixty-seventh, Sixty-eighth, and Sixty-ninth Congresses. Hawes resigned before completing his third term in the House, stepping down on October 15, 1926. The following month he was elected to the Senate. Because Senator Selden P. Spencer had died in office, Hawes took his Senate seat three months early, on December 6, 1926, replacing interim appointee George H. Williams. As Senator, Hawes worked for better flood control. This tied in with his earlier involvement with the Lakes to the Gulf Waterway Association when his "Missouri Plan" for levees along the Mississippi River was passed by Congress in 1929. An avid outdoorsman, he also supported efforts in wildlife conservation and was appointed to the Migratory Bird Conservation Commission in 1929. Senator Harry Hawes best-known achievement in Congress was the legislation that bears his name, the Hare–Hawes–Cutting Act. Created in conjunction with Representative Butler B. Hare of South Carolina and New Mexico Senator Bronson M. Cutting, the act aimed to grant the Philippine Islands full independence in graduated steps over a ten-year period. The legislation passed Congress in December 1932, but was vetoed by President Herbert Hoover. When Congress resumed work after the holiday break they overrode the veto on January 17, 1933. However one prerequisite of the act was ratification by the Philippine Senate, which failed to happen. The next year, 1934, a second effort very similar to the Hare-Hawes-Cutting act, the Tydings–McDuffie Act was finally agreed