The construction of Sioux City Army Air Base began in March 1942, about three months after the Japanese attack on Pearl Harbor. Opened on July 5, 1942, it became a major training center during World War II for crewmen of B-24 Liberators and B-17 Flying Fortresses. With the end of World War II, the former training base switched to becoming a processing center to discharge personnel out of the service and back into civilian life. Sioux City Army Air Base closed in December 1945, but in September 1946, the airfield was opened by the Air Force Reserve. Sioux City Air Base was one of the first Air Force Reserve bases established after the war, and in December 1946, the 185th Iowa Air National Guard unit was established at Sioux City. Assigned to the new Air Defense Command (ADC) upon reactivation, the 140th Army Air Force Base Unit was activated as its host organization. The mission of the 140th AAFBU was to offer flight and ground training to all commissioned and enlisted members of the Air Force Reserve residing in Iowa, Minnesota, Nebraska, South Dakota and Wyoming. By the mid-1960s, ADC was reducing its forces, and on April 1, 1966, the 31st AD was reassigned and the airport was turned over to the Air Force Reserve and Iowa Air National Guard for limited military use. The airport designator "SUX" has been contentious, due to the name sounding identical to the slang word "sucks". Sioux City Mayor Craig Berenstein in 2002 described SUX as an "embarrassment" to the city. After petitioning the FAA for a changed airport identifier in 1998 and 2002, authorities found the alternatives offered - GWU, GYO, GYT, SGV, and GAY - to be unappealing, and elected to stay with SUX. In October 2007, airport board member Dave Bernstein proposed embracing the identifier, saying "Let's make the best of it. I think we have the opportunity to turn it into a positive," and noting "I've got buddies that I went to college with in different cities that can't even remember their own birthdays, but they all know the Sioux City designator — SUX." The airport now sells merchandise with the words "Fly SUX". For several years the only airline service was from Northwest Airlines, commuter service on Northwest Airlink to Minneapolis–Saint Paul International Airport. In October 2007, Frontier Airlines began service with two daily flights between its Denver hub and Sioux City. The new service from Frontier quickly expanded to three daily non-stop flights to Denver and helped passenger traffic increase by over 150%. In April 2008, Frontier announced it would be ending service to Sioux City on May 12, citing economic conditions after the company filed for Chapter 11 bankruptcy. In July 2011, Delta Air Lines (who merged with Northwest in 2009) told the United States Department of Transportation that a weak economy and lower seat demand would force the company to reduce flights to several smaller communities including Sioux City unless it received assistance from the federal government. Delta said planes left Sioux Gateway with an average of only 51.4% of their seats full, so given that the company planned to remove turboprop planes and some of its 50-seat jets from service by the end of 2011 it wasn't economically viable for Delta to continue service. Of the average 75 passengers who flew the SUX-MSP route each day, only nine flew directly to the Twin Cities or to Sioux City, according to the Metropolitan Airports Commission. The majority of passengers were connecting through MSP to destinations such as Chicago, Phoenix or Orlando. Delta made its final flights into Sioux Gateway in April 2012. American Airlines announced on December 9, 2011, that their subsidiary, American Eagle, would begin serving Sioux City from Chicago O'Hare in the spring of 2012. American ended service to Sioux City on April 5, 2021.