A longstanding regional rivalry lies behind the web of conspiracies that US authorities on Tuesday said linked Iranian agents to a plot to kill the Saudi Arabia ambassador in Washington.

Few contests have defined the modern Middle East like that between petroleum power Saudi Arabia, ethnically Arab and Sunni Muslim, and the Islamic Republic of Iran, which is ethnically Persian and Shiite Muslim.

The Shiite-Sunni divide stretches back 14 centuries; the conflict between the Persian Empire and its Arab neighbors many centuries before that.

But the modern incarnation of the regional struggle casts Iran as leader of an anti-US, anti-Israeli "Axis of Resistance" that includes Syria and backs Hezbollah in Lebanon and Hamas in the Gaza Strip.

The Saudi camp is more pro-Western, includes rich Persian Gulf monarchies, and backs Fatah in the West Bank and has posited a peace plan that would accept the Israeli state if it withdrew to 1967 borders.

The regional battle is engaged at every level, from diplomacy to nuclear ambitions. Saudi officials have told their American counterparts that if Iran one day acquires a nuclear weapon, it will feel it also needs one.

But Wikileaks revelations from a 2008 meeting between Saudi Ambassador to Washington Adel al-Jubeir – the man who US officials said Tuesday was the target of the alleged Iran-linked assassination plot – and American diplomats indicate that Riyadh had much more strategic aims.

Secret US Embassy cables from Riyadh indicate that King Abdullah of Saudi Arabia repeatedly urged the US to attack Iran to stop its nuclear program.

One cable quoted Mr. Jubeir, who “recalled the King’s frequent exhortations” to end Iran’s nuclear program. Jubeir is quoted as saying: “He told you to cut off the head of the snake.”

The permutations of this rivalry have spiked repeatedly in recent decades. Saudi Arabia accuses Iran of the 1996 Khobar Towers explosion, which killed 19 US servicemen in the eastern city of Dhahran.

Though Iran has not invaded another country for 200 years, fear of Iran's regional ambitions – Ayatollah Ruhollah Khomeini insisted that Iranians "export" their revolution worldwide – has prompted Saudi Arabia to lead the Gulf states with billions of dollars of weapons purchases from the United States alone, to "defend" themselves from potential Iranian predations.

Saudi-owned Patriot anti-missile systems ring the regions, ostensibly to stop any Iranian missiles attack, and Saudi Arabia and other key US allies in the region today operate under a nuclear umbrella agreement – even though Iran is years from building a nuclear warhead, if it were to chose to do so.

The Arab Spring revolutions that have toppled dictators this year in Tunisia, Egypt, and Libya have been welcomed as an "Islamic Awakening" (except in Syria) by Iran's leadership – despite an obvious weakening of its own soft power across the region, after violently crushing its own pro-democracy protests in 2009.

But the street protests in one country after another have also thrown the Saudi-Iran rivalry into clear view. When Saudi Arabia sent 1,000 troops across the causeway at the request of Bahrain's embattled king – who had no other way of stopping the tiny emirate's pro-democracy uprising, largely led by Bahrain's disenfranchised majority Shiites – Iran reacted with alarm, and threatened to mobilize Iran's own Revolutionary Guards.