1980 Jimmy Carter - Carter Doctrine

Beginning in World War II, the Persian Golf had become the leading producer of oil, and supplied most of the oil for the Allied Armies. It remained a strategic concern throughout the cold war. Then, at the end of December 1979, the Soviet Union sent thousands of troops into Afghanistan and assumed control of Kabul and other large portions of the country. With Afghanistan containing more than two-thirds of the worlds exportable oil, President Carter saw this as a grave threat to the movement of Middle East oil and to the security of the U.S.

In the 1980 State of the Union address, President Carter opened the door to military action against the Soviet Union in what would later become known as the 'Carter Doctrine':

"The Soviet Union is now attempting to consolidate a strategic position, therefore, that poses a grave threat to the free movement of Middle East oil. This situation demands careful thought, steady nerves, and resolute action, not only for this year but for many years to come ... Let our position be absolutely clear: An attempt by any outside force to gain control of the Persian Gulf region will be regarded as an assault on the vital interests of the United States of America, and such an assault will be repelled by any means necessary, including military force."

http://www.presidency.ucsb.edu/ws/index.php?pid=33079
http://en.wikipedia.org/wiki/Carter_Doctrine
https://history.state.gov/milestones/1977-1980/soviet-invasion-afghanistan
http://media-2.web.britannica.com/eb-media/67/127567-050-34A0D945.jpg