A Memory of The World's Fair: Expo '74
"Expo '74 is notable as being the first world's fair with an environmental theme. The majority of world's fairs since Expo '74 have had environmentalism as part of their themes."

Also:

"This world's fair was particularly notable for its large participation by the Soviet Union, the first time since World War II that they participated in a United States world's fair. It would also be the last time."

http://www.expomuseum.com/1974/

Full-Res: 11x17 in. @ 300 DPI