Increasing production and declining consumption of oil and gas have unexpectedly brought the United States markedly closer to a goal that has tantalized presidents since Richard Nixon: independence from foreign energy sources, a milestone that could reconfigure American foreign policy, the economy and more. In 2011, the country imported just 45 percent of the liquid fuels it used, down from a record high of 60 percent in 2005.

How the country made this turnabout is a story of industry-friendly policies started by President Bush and largely continued by President Obama — many over the objections of environmental advocates — as well as technological advances that have allowed the extraction of oil and gas once considered too difficult and too expensive to reach. But mainly it is a story of the complex economics of energy, which sometimes seems to operate by its own rules of supply and demand.