The allegations of a plot to kill the Saudi ambassador to Washington are the latest and perhaps most audacious eruption in the simmering feud between Iran and Saudi Arabia, two regional powers that have long waged proxy battles for influence in the Muslim world.

The two countries have been locked in a cold war for decades, especially since the 1979 Iranian revolution established a theocracy in Tehran that has openly challenged the legitimacy of the royal House of Saud. The rivalry has been fueled by sectarian tensions — Iran has a predominantly Shiite Muslim population, while Saudi Arabia is mostly Sunni — but also centers on their respective ambitions to exercise political and economic power throughout the Middle East.

The conflict has waxed and waned over the years but flared up with renewed intensity during the Arab Spring, which ignited popular uprisings that have toppled or threatened to unseat longtime allies of both countries.

Officials from both nations wasted no time in flinging acrimonious insults in the aftermath of the Justice Department’s announcement that it had charged two Iranians with conspiring to murder Adel al-Jubeir, the Saudi ambassador to the United States and a confidant of Saudi King Abdullah.

The Saudi Embassy in Washington said the plot is “a despicable violation of international norms, standards and conventions and is not in accord with the principles of humanity.” Meanwhile, a spokesman for the Iranian Foreign Ministry denied any government involvement, calling the criminal accusations the fruits of a U.S.-Israeli “conspiracy” to isolate Tehran.

“Hell will break loose,” said Hilal Khashan, professor of political science at the American University of Beirut. “I don’t expect war to break out tomorrow, but if there was any hope that Saudi-Iranian relations would improve, this will be the end of it.”

A rivalry intensifies

Tensions have been high since March, when the Saudis sent troops into neighboring Bahrain to prop up the Sunni royal family there amid fears that Shiite demonstrators might ally themselves with Iran. Meanwhile, Tehran has sweated over a popular challenge to the rule of the Assad family in Syria, Iran’s closest Arab ally.

One possible outcome of the reported plot is that the Saudis will retaliate by boosting overt support for the protest movement in Syria. “The Saudis were reluctant to commit themselves against the Syrian regime,” Khashan said. “But now they will become more audacious.”

Saudi Arabia and Iran have also vied for influence in Iraq ever since 2003, when the U.S.-led invasion ousted Saddam Hussein but also ignited a sectarian civil war. The two rivals have likewise jousted for influence in Lebanon and the Palestinian territories.

“The gloves are off, I think,” said Frederic Wehrey, a Rand Corp. analyst who has studied Saudi-Iranian relations. “We were already at this low point, and this will only make things worse.”

Iran’s ruling clerics see the Saudi royals as corrupt custodians of Islam’s holiest shrines. In turn, Saudi Arabia is convinced that Iran harbors unchecked ambitions to dominate the region, a fear manifested in suspicions that Iran is trying to develop nuclear weapons.

Saudi Arabia’s oil-rich monarchy has felt especially threatened by Tehran since the 1979 Iranian revolution toppled Shah Mohammad Reza Pahlavi and replaced him with a theocratic government. But those Arab fears are rooted in history, dating to the days of Persian empires.

“They believe that there’s a Persian impulse for regional hegemony that they’ve been struggling against not just for the last 30 years, but for hundreds of years,” said Jon B. Alterman, director of the Middle East program at the Center for Strategic and International Studies.

At the same time, Alterman and other analysts predicted that officials in Riyadh would stop short of severing diplomatic relations with Tehran.

“The Saudi instinct is never to cut people completely off,” Alterman said. “Will there be repercussions? Yes. But Saudi Arabia is not going to close the door on rapprochement in the future.”

Memories of a past plot

The charges announced by the Justice Department on Tuesday revived memories of another plot to kill Saudi diplomats two decades ago.

In 1990, three Saudi diplomats posted to Thailand were gunned down in Bangkok on the same day. A year earlier, a Saudi business executive in Bangkok was also shot to death.

A Shiite group in Beirut with links to Iran asserted responsibility for the businessman’s slaying. Some U.S. and Thai officials have said that the killing of the diplomats was the result of a Saudi feud with Hezbollah, the Lebanese militia that draws support from Iran.

Iran was accused of orchestrating a series of terrorist attacks against diplomatic and political targets in the ensuing decade, including the 1992 bombing of the Israeli Embassy in Buenos Aires and a 1996 truck bombing in Dhahran, Saudi Arabia, that killed 17 U.S. troops at the Khobar Towers housing complex.

In recent years, however, Iran has looked to traditional battlefields, supplying weapons and explosives to insurgents fighting U.S. forces in Iraq and Afghanistan.

In that regard, if the allegations are true, the contours of an amateurishly brazen plot to kill the Saudi ambassador with a bomb at a restaurant in Washington mark a change in tactics for Iran, analysts said.

“It is sort of out of keeping with normal standards of Iranian subtlety,” said Simon Henderson, a Saudi specialist at the Washington Institute for Near East Policy.

Sly reported from Beirut.