During the Second World War, the Royal Air Force and the United States Army Air Force constructed many airbases in East Anglia for the heavy bomber fleets of the Combined Bomber Offensive against Nazi-occupied Europe. East Anglia was ideally suited to airfield construction, as it includes large areas of open, level terrain and is close to mainland Europe. Many of the airfields can still be seen today, particularly from aerial photographs, and a few remain in use, the most prominent being Norwich International Airport. Pillboxes were erected in 1940 to help defend the nation against invasion, and they can also be found throughout the area at strategic points.