The correct answer to this open question is the following.
The United States was too dependent on oil produced in other countries.
One good example of this US dependency of oil produced in foreign countries was the oil crisis in 1973 and 1979. The United States faced gas shortages in those years because the OPEC disrupted oil supplies. OPEC stands for Organization of Petroleum Exporting Countries. In those years, American citizens had to withstand long lines in gas stations to get some gasoline.
