A MERICA HAS been a superpower for decades. As a superpower in global energy markets, however, it is barely an adolescent. As recently as 2015 it was illegal to export oil. Within ten years the shale boom has transformed it into the world’s biggest producer of crude. No longer must it tiptoe around regimes whose policies it detests but whose oil it craves. President Donald Trump touts an age of “energy dominance”. He has put its burgeoning energy prowess to the test with sanctions on Iran and Venezuela. But dreams of dominance are running into the realities of energy markets.

On April 22nd the American administration announced that no further waivers would be granted to countries importing oil from Iran. “We are going to zero,” declared Mike Pompeo, the secretary of state. Waivers to eight countries, granted in November, were due to expire on May 2nd. Even so, investors were shocked that no exceptions were allowed. According to the state department, Saudi Arabia, the United Arab Emirates and America will help meet demand. But Brent crude, the international benchmark, quickly topped $74, the highest level in nearly six months.

The Trump administration is right to make to make a fuss about America’s oil boom. According to the International Energy Agency, by 2021 the country may be a net exporter of oil. This would be a stunning reversal. Not long ago, notes Amy Myers Jaffe of the Council on Foreign Relations, a think-tank, oil imports were the largest cause of America’s current-account deficit.

But it is wrong to think it can bring about dramatic change in Iran and Venezuela without risking dramatic spikes in petrol prices. (Those concerned about climate change might welcome expensive oil; Mr Trump is not one of those people.) America’s government does not control its oil industry. Oil firms are backed by investors whose interests do not necessarily align with those of the president. Nor can it control the world’s many buyers and sellers of crude. American production accounts for about 15% of global output, a striking increase from what it was but still a small share of total supply. Complicating matters, investors have struggled to anticipate the Trump administration’s actions. Rather than stabilise oil markets, America has been as likely to do the opposite.

Mr Trump announced last May that America would impose sanctions on Iran and withdraw from the deal on international oversight of its nuclear capacity signed during Barack Obama’s presidency—the “worst deal ever”, as Mr Trump liked to call it. He urged the Organisation of Petroleum Exporting Countries ( OPEC ), privately and on Twitter, to boost production to help restrain oil prices. Saudi Arabia duly did so, increasing output by 600,000 barrels a day from June to November, when the sanctions were to take effect.

At the last minute, however, Mr Trump announced that eight countries would be exempt from the waivers for six months—including China and India, the biggest importers of Iranian oil. Investors and Saudi Arabia were caught off guard. The kingdom requires oil at around $80 a barrel to meet its budgetary needs; in December Brent prices sank to $51. OPEC and its allies scrambled to cut production and shore up prices. In December they said they would decrease output by 1.2m barrels a day.

The next sanctions came in January, after opponents of Nicolás Maduro’s ruinous regime in Venezuela declared Juan Guaidó, the head of its national assembly, the legitimate leader. America recognised Mr Guaidó and unveiled sanctions to choke off the country’s oil company, Mr Maduro’s main source of cash. “America now appreciates that energy is a source of foreign-policy strength rather than a vulnerability,” says Meghan O’Sullivan, a former adviser to George W. Bush and a professor at Harvard University.

But using that strength effectively can be difficult. Sanctions on Iran and Venezuela have yet to produce the desired effect. Mr Maduro still clings to power. Iran has met none of Mr Trump’s demands. Its crude exports have fallen by about half since Mr Trump said America would withdraw from the nuclear deal last year, but they remained well above zero in March, at about 1.4m barrels a day, according to Kpler, a data firm. China, in particular, has continued to import Iranian oil (see chart). The announcement that waivers will expire seeks to change that. Countries that continue to import Iranian oil, the state department says, could be cut off from America’s banking system.