The old Republican rallying cry “drill, baby, drill” was supposed to be simplistic sloganeering masquerading as policy.

It turns out that it represented transformative wisdom. The fall in the price of oil — about 40 percent in the past several months, down to less than $70 a barrel — is largely the result of the U.S. drilling, and then drilling some more, baby.

Big drops in the price of oil usually accompany recessions and are caused by declining demand. Not this one. Lackluster demand from Europe and China is a factor, but the driver is the American shale boom that is perhaps the most wondrous national achievement of the past decade.

No one would have predicted it. To the contrary, experts predicted the opposite. In 2008, the International Energy Agency was projecting U.S. production would decline or remain flat for decades. Prior to the recession, the price of oil peaked at nearly $150 a barrel, and with global demand rising, it looked like it would remain at an elevated level forevermore.

But now the U.S. is producing more than 3 million barrels a day than it did several years ago. As Robert Bryce of the Manhattan Institute points out, this is like adding another Kuwait to world oil production. The Marcellus Shale in Pennsylvania alone, he writes, has added another Iran to world natural-gas production.

Perhaps President Barack Obama can be forgiven for not understanding the consequence of this, given his attenuated understanding of complex market forces — like supply and demand. As recently as 2012, he was confidently asserting that “we can’t just drill our way to lower gas prices.” Drivers enjoying the $1 drop in the price of gas since May might beg to differ.

The lower price of oil is an almost unalloyed good. A recent Wall Street Journal story was headlined “Tumble in Oil Prices Spurs New Bets on Global Growth.” Who can quibble with higher projections of global growth, including in the United States?

The drop in the price of oil, and the resulting reduction in the price of energy, is a boon to American consumers. The fall in gas prices puts tens of billions more dollars in their wallets.

It is a boon to the working class. Households making less than $50,000 a year, according to The Wall Street Journal, were spending more than 20 percent of their after-tax income on energy in 2012. The number had been only 12 percent in 2001.

It is a boon to industry. It reduces the costs of manufacturing and transport.

It is a boon to automakers. Lower gas prices are helping drive a surge in sales of trucks and SUVs that are more profitable for Detroit than small cars.

It is a boon to agriculture. It reduces the price of plowing and harvesting, and makes fertilizer cheaper.

All of this is in keeping with the great truth that cheap, abundant energy has always contributed immeasurably to American prosperity. That makes it all the more perverse that the left wants to make energy more expensive in its war on fossil fuels, push for ever-more-stringent environmental regulations and quixotic campaign against global warming. Environmentalists rue that the decline in energy prices makes alternative energy even more uneconomical.

According to the Institute for Energy Research, “Nearly every barrel of new U.S. oil production can be attributed to the use of horizontal drilling and hydraulic fracturing technologies.” The left hates fracking and would love, if it could, to hamstring it.

The Obama administration is pushing new regulations on everything from coal-fired power plants to, perhaps soon, methane. If it had its way, it would impose a tax on carbon, and it wants to lead the world in restricting the use of fossil fuels in the name of combating climate change.

The assumption of the administration has always been that fossil fuels represent the past, when they are, in reality, powering the American economy into the future. “Drill, baby, drill” indeed.

Rich Lowry is editor of the National Review.