Vali Nasr writes: The latest warning by Iran, that a U.S. aircraft carrier that recently transited through the Strait of Hormuz should not do so again, is a sign to the West that should be well-observed. It tells us the regime in Tehran is ready for a fight.

Tensions between Iran and the U.S. are so high, a conflagration could be tripped off without either country intending it. This latest spiral of hostility began after the U.S. and its European allies responded to the International Atomic Energy Agency’s report on Iran’s nuclear activities by imposing and threatening additional, tougher sanctions. New U.S. measures may drastically cut Iran’s oil revenue.

That, in turn, may threaten the Iranian regime’s hold on power. Predictably, then, the ruling clerics are responding with shows of strength to boost solidarity at home. And they can be counted on to accelerate Iran’s nuclear program, which they see as a deterrent to foreign intervention.

To escape this self-defeating outcome, the Western powers should imagine how the situation looks from Tehran.

In recent months, Iranian protesters have brazenly attacked the U.K. Embassy in Tehran. Iran has claimed to have downed a U.S. drone, put on 10-day war games simulating attacks on U.S. ships, and threatened to push oil prices to $250 a barrel and to close the Strait of Hormuz, through which about 20 percent of all oil trade passes.

This defiance marks a change. Until recently, Iran had absorbed economic pressure from abroad. It had remained silent in the face of covert operations aimed at slowing the progress of its nuclear program, brushing off the destructive Stuxnet computer worm, apparently a joint U.S.-Israeli project. But the government has been embarrassed and unnerved by multiple assassinations of its scientists and by suspicious explosions at its military facilities. One blast killed the general charged with developing Iran’s missile program. The attacks have shaken the country’s security forces.

The ruling clerics are also worried about the impact of economic sanctions, which have greatly reduced Iran’s access to global financial markets, created shortages of imported items, and increased inflation and unemployment. The rial has fallen to its lowest point against the dollar, and capital is fleeing the country at an alarming rate. The government has been forced to scrap numerous infrastructure projects, especially in the oil- and-gas sector.

These hardships have caused popular discontent. The next set of sanctions may bring street protests. Iran’s rulers fear a repeat of the demonstrations of 2009. They now see the U.S. policy on Iran — of toughening sanctions and also, at the United Nations, addressing Iran’s human-rights record and support for terrorism — as one aimed at regime change.

Meanwhile, The Guardian reports: European governments have agreed in principle to impose a ban on imports of oil from Iran, a potentially serious blow to the already unsteady Iranian economy and a significant escalation in the international pressure on the Tehran government.

Negotiations on a European oil embargo on Iran have been under way since an EU foreign ministers meeting last month. On Wednesday night a European diplomat said there was now a consensus that the ban on crude imports would be applied, but that there was still debate on the timing and duration of the measures.

“There is agreement in principle on Iranian oil imports,” the diplomat said. “What is now being discussed is what exceptions there would be for existing contracts and what kind of review clause there will be.”

The formal agreement on the measures is due to be finalised by the next EU foreign ministers meeting on 30 January, but the sanctions will probably not take effect immediately. One diplomat said they could come into force at the same time as US financial sanctions aimed at international banks financing Iranian crude oil sales.