These two main sects share many beliefs and practices but also differ, with the split between them originating from the death of the Prophet Mohammed in the year 632 AD.
Sunni, who make up 85 per cent of the world’s muslims, believe the Caliphs chosen from among his close Companions were the rightful successors.
The Shia venerate the fourth Caliph – the Prophet’s cousin and son-in-law, Imam Ali – and Ali’s son Hussein, who was defeated and killed in a bid to take leadership for himself.
Since 1979 and the revolution in Iran, militant Shia theology has seen a renaissance. It has been assisted by the aggressive western response to the rise of jihadism and the 9/11 attacks. Al-Qaeda and its off-shoots are Sunni supremacist organisations, and their rise has split the Sunni world.
In Iraq, a Sunni minority ruled over the Shiite majority for years, until the allied invasion overthrew Saddam Hussein, who was Sunni. With democratic elections, Shia parties were able to dominate the new politics.
In Syria, Sunni Muslims are in a large majority, but the country has been ruled for the last 45 years by members of the Alawite sect, a Shia off-shoot. The Iranians have become the backers of last resort for the Assad regime, leading to a massive strengthening of Shia influence in the country, even though non-Alawite Shia make up less than five per cent of the population.
Increasingly, the diplomatic clash across the region is led by Sunni Saudi Arabia and Shia Iran. But on the ground, the war is between Hizbollah and similar Shia militias, and Al-Qaeda and other Sunni militants. Governments choose between them – or find themselves caught between the two.