The United States has re-imposed punitive measures targeting the Iranian oil and financial sectors in the toughest sanctions ever placed against Iran.

The measures that take effect are the most concrete result yet of US President Donald Trump's controversial decision in May to abandon the multi-nation nuclear deal with Tehran.

The sanctions are the most damaging to the Iranian economy - targeting its oil sales, its wider energy industry, shipping, banking, insurance and so on.  The  sanctions are intended to apply pressure on other countries to prevent them trading with Tehran.  The idea is to dissuade them from purchasing Iranian oil, which brings in a huge proportion of Iran's revenue. In addition, sanctions will be imposed upon hundreds of named entities and individuals.

The sanctions will directly affect companies from third countries doing business with Iran.  World oil markets have been on alert, nervously set to gauge the consequences of the sanctions.  Some experts consider that the sanctions could upset world oil markets, though the US has granted temporary waivers to eight jurisdictions to continue importing Iranian oil.

The Japan Times notes that six months after U.S. President Donald Trump bolted from a nuclear deal on Iran, the United States tries to strangle the country’s economy with sweeping sanctions, but doubts abound on how effective the campaign will be.

Iran has, according to U.N. inspectors, abided by the 2015 agreement, which is still supported by European powers and Russia and China, which all signed the nuclear deal.

“This is not 2012, when the world was united behind sanctions against Iran.  This is the Trump administration trying to force the rest of the world to go along with a policy that most countries do not accept,” said Barbara Slavin, an Iran expert at the Washington-based Atlantic Council.

“The U.S. has had some success in terms of frightening away major corporations. The sanctions hurt a lot. But Iran is still going to be able to sell oil,” especially to China, she said.

The United States has accepted that it will need to issue waivers to countries that do not fully stop buying Iranian oil, with friends of the United States such as India and South Korea looking for sanctions exemptions, and Tehran may keep up clandestine sales.

A senior official from the Trump administration said last week that the U.S. has agreed to let eight countries — including Japan, India and South Korea — keep buying Iranian oil, but only temporarily.

The European Union has gone so far as to protect businesses that operate in Iran.  It has announced plans for a legal framework through which firms can skirt U.S. sanctions, although few major corporations have been eager to risk the wrath of penalties in the world’s largest economy.

U.S. Secretary of State Mike Pompeo has issued a list of demands for Iran that go well beyond the nuclear program that was the focus of Obama’s deal.

He wants the Shiite clerical regime to withdraw from war-ravaged Syria, where it is a critical ally of President Bashar Assad, as well as to end long-standing support to regional militant movements Hezbollah and Hamas.

Pompeo has also insisted Iran cut off backing for Yemen’s Houthi rebels, who are facing a U.S.-backed air campaign led by Saudi Arabia.

In a recent tweet Pompeo crowed that the International Monetary Fund is predicting a 3.6 percent contraction of Iran’s economy next year.

“That’s what happens when the ruling regime steals from its people and invests in Assad — instead of creating jobs for Iranians, they ruin the economy,” he said.

But experts see no rapid turnaround from Iran’s leaders — especially the military and clerical establishments.