The execution of Nimr al-Nimr, a Shi’a cleric and bitter opponent of the Saudi regime who regularly and publicly insulted the royal family, has triggered an unprecedented crisis between Tehran and Riyadh.Though it was not totally unexpected given the present geopolitical turmoil in the Middle East, the roots of the crisis are to be found in the long-standing feud between Sunni and Shi’a, which dates from the very beginning of Islam.Prophet Mohammad had wanted all Arab tribes to remain united, but the battle for his successor left Islam torn between Sunni and Shi’a, though both believe in the prophet and in the Koran and aspire to impose the rule of Islam on the entire world. Each developed their own narrative and their own ethos, which leaves no room for compromise or reconciliation.Following historical ups and downs, Sunni Islam, with Saudi Arabia as its leader, today accounts for 85 percent of all Muslims while Shi’a Islam, spearheaded by Iran, musters the remaining 15%.The Sunni block, however, is no longer monolithic.There are a number of radical organizations – from al-Qaida to Islamic State and some 40 smaller groups – aiming to use force to restore the caliphate through jihad. They are generally lumped under the name of Islamism or jihadist radical Islamic organizations though, like main stream Sunnis, their teachings are based on the Shari’a, perhaps professing stricter observation.Meanwhile, in Iran, Ayatollah Ruhollah Khomeini, who came to power in 1979, launched a new drive to impose Shi’a Islam on the whole Middle East as a first step to be followed by a world takeover. He called on Shi’a minorities in Sunni states to act against these states to destabilize them from within and eventually topple them and set up a Shi’a regime in their stead, thereby securing Iran’s position as regional power.Syria, ruled by the Allawis and hitherto shunned by mainstream Sunni, was given legitimacy by the Ayatollahs and became Teheran’s willing ally.Building on the frustrations of the Shi’a in Lebanon, which complained of discrimination, Iran set up the Hezbollah with a three-pronged objective: taking over the country, threatening Israel and developing subversive activities in Jordan and Egypt.In 2008, Egyptian authorities exposed a Hezbollah cell that planned an attack on the Suez Canal; today Hezbollah fighters are helping Assad in Syria at Tehran’s bidding.In the Gulf, Saudi Arabia is the main bulwark of Sunni Islam against Iran’s subversive activities and, as such, is considered that country’s arch-enemy.The kingdom has a number of unassailable assets. Both Islam’s holiest sites – Mecca and Medina – are situated in its territory; it has the largest oil reserves in the world and it is – or was – both friend and ally of the United States.Once again, Tehran resorted to subversion, inciting Shi’a minorities in the area. Iran did not hesitate to proclaim that Bahrain, where there is a Shi’a majority though the country is ruled by the Sunni Al Khalifa family, was the country’s 14th province. Egypt’s then president Hosni Mubarak rushed to Bahrain’s capital Manama to demonstrate to the Iranians that security in the Gulf was an essential component of Egyptian national security.In 2011, soon after he was ousted during the so-called Arab Spring, violent manifestations threatened to topple the regime in Bahrain, as well. Saudi Arabia and other Emirate countries sent troops to help quell the revolt.There is a significant Shi’a minority in Saudi Arabia, mainly in the Eastern part of the country where the largest oil fields are situated and Iran spared no efforts to enlist its help. Nimr al-Nimr was its firebrand leader and, as such, was jailed a number of times.In 2012, he fomented several demonstrations against the regime, was jailed again and sentenced to death for rebellion under the strict Shari’a laws backed by a number of verses of the Koran. His execution was intended as a wakeup call to the Sunni states and as a warning to the Shi’a minority and to Iran – henceforth Riyadh would no longer tolerate Iranian subversive activities and threatening declarations against the kingdom and its Emirate allies.Threats that are taken seriously in view of Iran’s actions in recent years – open intervention in Syria through its proxy Hezbollah to help Assad; furthering its influence in Iraq by reinforcing Shi’a political parties but also by setting up Shi’a militias to replace the country’s armies, which failed dismally in their fight against Islamic State.Then came the last straw.Tehran spurred on Houthi rebels in Saudi Arabia’s southern neighbor, Yemen, which commands the entrance to the Red Sea and the Suez Canal, supplying them with weapons and ammunitions. The Saudis felt they were surrounded.If the situation was not dire enough for the embattled kingdom, the desertion of the United States, its staunchest ally for decades, left it with no option but to take a stand.Under the leadership of President Barack Obama, secret talks had been held with Iran, leading to an agreement purportedly delaying the manufacture of nuclear weapons by a number of years but not addressing the country’s terrorist actions against neighboring states – s move seen in the Middle East as de facto recognition by Washington of Iran’s and Shi’a hegemony.Battle lines are drawn.Saudi Arabia – which unlike Tehran is taking part in the American led coalition against Islamic State has set up a coalition of its own to fight the Houthis in Yemen.In December, it launched another coalition – Islamic countries against Islamic State.It is making an all-out effort to help Egypt’s economy. Under Mubarak, Cairo was at the forefront of the fight against Iran’s attempts at hegemony.Here, too, America has more or less turned its back on its former great ally. Cairo is shifting its stand toward Russia, which supports Assad. A minor problem Egyptian and Saudi leaders are doing their best to ignore.What now? Saudi Arabia has broken off ties with Tehran, followed by Bahrain, Sudan and Djibouti – two countries that have suffered greatly from Iranian subversive activities.The United Arab Emirates downgraded their ties. Kuwait and Qatar recalled their ambassadors. Sunni countries are temporarily setting aside their quarrels and interests to face the common Shi’a enemy.An urgent meeting of the Arab League will be convened this week. And the United States? It is asking both sides for restraint, which is rather meaningless, especially considering that it was because it sided with Tehran and weakened its erstwhile allies that the present situation has developed.Russia is also offering its good offices to defuse the crisis.Another success for Putin, who is asserting his country’s greater influence in the Middle East.It seems that Tehran, embroiled in fighting Islamic State and in sustaining Bashar Assad’s regime, has no wish to add fuel to the fire. So called moderate voices in Iran openly accuse the regime of having overreacted in letting frenzied mobs loose on Saudi representations.The present crisis may end in a suitable compromise – but the age old enmity between Sunni and Shi’a remains stronger than ever.The writer, a fellow of The Jerusalem Center for Public Affairs, is a former ambassador to Romania, Egypt and Sweden.