The long-running crisis between Iran and the West, particularly Israel and the U.S., has reached a new and dangerous level of intensity. This week an important Iranian nuclear scientist was assassinated in Tehran. At the same time, European oil refiners have stopped spot purchases of Iranian crude oil in anticipation of a European Union meeting later this month that might well impose a total oil embargo on Iran. That's serious, and Iran knows it.

The killing of 32-year-old Mostafa Ahmadi-Roshan by a magnetic bomb attached quickly and openly to his car by two assailants passing by on motorcycles seems to be part of a larger plan to kill off key Iranian nuclear and military figures. Ahmadi-Rosan was deputy head of Iran's Natanz nuclear-enrichment facility, one of the plants in which the West fears Iran is developing the capability to produce nuclear weapons.

Last year, Massoud Ali Mohammadi, a professor of physics and nuclear energy at Tehran University was killed by a remote-controlled bomb attached to his car. In fact, several people involved in the Iranian nuclear program have been targeted since 2007. In November 2010, Majid Shahriyari, a physics professor, was killed and Fereydoon Abbas-Davani, another nuclear scientist, was injured in an attack similar to the one this week that killed Ahmadi-Roshan. Abbasi-Davani now heads the Iran Atomic Energy Organization, and by all accounts is not well disposed toward Israel and the U.S., which he blames for the attack that killed his colleague and almost killed him.

The Iranian government blames Israel and the United States for the assassinations as part of an escalating covert war to sabotage Iran's nuclear program. The Iranian accusations gained ground after the unexplained blast at a military site close to Tehran last November, the latest in a series of setbacks and mysterious deaths in the country's nuclear program. Tehran also sees the spread last year of the Stuxnet computer virus that affected some of the centrifuges Iran's nuclear program uses to enrich uranium as part of the covert war.

Israel, of course, either disclaims any knowledge of the assassinations or simply refuses to comment, a standard approach in dealing with sensitive intelligence matters. This week, the U.S. flatly declared it had nothing to do with the assassination of Ahmadi-Roshan. Iran, of course, is convinced that both countries are involved, especially Israel, whose Mossad intelligence agency is without doubt the best and most effective in the Middle East.

It seems probable, though not yet provable, that Israel is deeply involved in these assassinations and other covert operations, if only because Israel has the most to lose in any nuclear standoff with Iran. Moreover, Israel is the only power in the region capable of carrying out these operations. Perhaps Washington is cooperating, perhaps not. The U.S. is either an active or silent partner in these operations. Iran knows it and is increasingly nervous. Hence the threat to close the Straits of Hormuz.

Iran is the world's third-largest oil exporter and ships about 2.3 million barrels a day, mostly to Asia. The European Union buys about 450,000 barrels per day, thus any EU decision later this month to impose a full oil embargo on Tehran would have extremely serious consequences for the Iranian economy. Washington has recently introduced sanctions to penalize foreign financial institutions that deal with Iran's central bank, which clears most of Iran's oil exports, though the impact of those penalties is not clear. Tehran does not have many current customers, other than the EU, China, India, Japan, South Africa, South Korea Sri Lanka, Taiwan and Turkey. Thus, if the EU goes ahead with a total oil embargo, then almost certainly Japan, South Korea, Taiwan and India will follow with some reductions of their own. Iran, according to oil industry experts, is looking at a possible reduction of 550,000-850,000 barrels per day, or some 25 percent to 35 percent of its total oil production. That is alarming news for Tehran and probably accounts for its more bellicose attitude recently. It feels under siege.

We need to remember that apart from the bellicosity of the present Iranian regime, Iran's resentment of the West is rooted not only in the often-offensive religious sentiments of the conservative mullahs, but also in comparatively recent history. It was the United States and
Britain that in 1953 got rid of Prime Minster Mohammad Mossadegh, possibly the most popular political leader in modern Iranian history. He was freely elected. In his place, Washington and London restored the shah. The national resentment over that act of foreign interference eventually led to the return of the Ayatollah Khomeni. The Iranians have never forgotten that episode. Neither should we.

Bill Stewart, a former Foreign Service officer and correspondent for Time magazine, lives in Santa Fe. He writes weekly on current affairs.