The Sunni insurgency raging through Shiite-ruled Iraq has added fresh fuel to a centuries-old feud.
This week, as al Qaeda-linked rebels known as the Islamic State of Iraq and al-Sham (ISIS) overran city after city, fears re-emerged that fighting between the two branches of Islam could send Iraq down the path of a sectarian civil war.
Sign up for breaking news alerts from NBC News
"The Iraq conflict plays out on several levels between Sunnis and Shiites. First and foremost, it's about how to share power in a 21st century state. The prime minister, a Shiite, has failed abysmally in creating a formula to share power with the Sunnis, the traditional political masters in Iraq," said Robin Wright, a joint fellow at the U.S. Institute of Peace and the Woodrow Wilson Center, non-partisan institutions.
Tension between other countries in the region has also stoked the fighting, which revives suspicions that have existed between Shiites and Sunnis that date back 1,400 years, she said.
But experts caution that the ancient history between the two sects is only one element of the current conflict in Iraq.
"I don't necessarily think there's a one-to-one correlation between the historic doctrinal differences and what is happening in Iraq right now," said Haider Ala Hamoudi, an associate professor of law at the University of Pittsburgh School of Law, who teaches on Islamic law. The fighting now, he said, arose more over a fight for power than the variations in beliefs held by Sunnis and Shiites.
What was the origin of the Sunni-Shiite split?
Just what are those variations? The divide is traced to 632 A.D., when the Islamic Prophet Muhammad died and a debate emerged about who should be his successor.
Both sides agreed that Allah is the one true God and that Muhammad was his messenger, but one group (which eventually became the Shiites) felt Muhammad's successor should be someone in his bloodline, while the other (which became the Sunnis) felt a pious individual who would follow the Prophet's customs was acceptable.
"The original schism between Islam's two largest sect was not over religious doctrine. It was over political leadership," Wright said.
Sunnis came to constitute the majority of Muslims worldwide, but in Iraq, they were a minority — a powerful one that subjugated the Shiite majority. When Saddam Hussein's Sunni regime collapsed during the U.S. invasion in 2003, it ended Sunni political dominance that had ruled Iraq since the end of World War I.
Later, when Shiite Nouri al-Maliki became prime minister in 2006, critics argued that he missed a crucial opportunity for peace by burning bridges with Sunnis instead of strengthening them.
Sunni insurgencies, fearing that the Shiites would take revenge now that they had power, took hold, and the Iraqi army struggled to stop them with no outside support after the U.S. withdrew all troops in 2011.
"With the lack of the United States there, you don't have anyone who can be a broker and work hard toward inclusion," Hamoudi said, adding that U.S. troops' time in Iraq wasn't able to resolve the Sunni-Shiite rift. "It was too short a time to overcome these decades of mistrust."
What do Sunnis and Shiites have in common?
Both Sunnis and Shiites read the Quran, the sayings of the Prophet. Both believe Prophet Muhammad was the messenger of Allah. And both follow the five tenets of Islam: They fast during Ramadan, pledge to make a pilgrimage to Mecca, practice ritual prayer (which includes five prayers each day), give charity to the poor, and pledge themselves to their faith.
Their prayer rituals are nearly identical, with slight variations: For example, Shiites will stand with their hands at their sides, Sunnis will put their hands on their stomachs.
They also both believe in Islamic law but have different applications for it.
Whereas Sunnis make up the majority of the Muslim world, from West Africa to Indonesia, the Shiites are centrally located, with a vast majority in Iran, predominance in Iraq and sizable populations in Syria, Lebanon and Yemen.
What are the differences between Sunnis and Shiites?
Their beliefs over who should have succeeded the Prophet Muhammad is the key theological difference between the two.
Sunnis also have a less elaborate religious hierarchy than Shiites have, and the two sects' interpretation of Islam's schools of law is different. Shiites give human beings the exalted status that is given only to prophets in the Quran, often venerating clerics as saints, whereas Sunnis do not.
But the fighting now boils down to a struggle for power, not theological doctrines. On Friday, Iraq's senior Shiite cleric urged all Iraqis to join the government's fight against the Sunni militants who have taken over large swathes of the country.
"People who are capable of carrying arms and fighting the terrorists in defense of their country ... should volunteer to join the security forces to achieve this sacred goal," Sheikh Abdulmehdi al-Karbalai said.
Nonetheless, old wounds still linger.
"Sunnis have always held power in Iraq in significant quantities," Hamoudi said. "Over the course of decades, through a series of revolutions, the decision to exclude Shi'a became much more conscious. They were feared as a group that could somehow sell the country to Iran.
"The exclusion of the Shi'a was not something that was just a historical accident, but was viewed as something that was important to preserve the state in its current form."