CHINOOK: An Army Boeing CH-47 Chinook hovers near delivered supplies at Fire Support Base Ripcord on July 18, 1970. The mountaintop base had been under North Vietnamese siege since July 1 and due to the terrain could only be resupplied by helicopter. In spite of Chinook crews’ efforts, the heavily outnumbered U.S. forces were ordered to evacuate on July 23. / Getty Images