The UK's ties with Saudi Arabia have come under growing strain in recent months over how to balance human rights concerns with the government's desire to promote a crucial trade and investment relationship. The Arab state sits on more than a quarter of the world's known oil reserves, making it one of the richest countries in the Middle East and a vital strategic partner to many Western nations. It is also home to the birthplace of the Muslim Prophet Muhammad and the cradle of Islam. Its rulers espouse a strict version of Sunni Islam known as Wahhabism. The Wahhabi interpretation of Islamic law includes harsh punishments such as public beheadings and restrictions on women. How did Wahhabism gain so much influence in the country? What, in turn, has been its effect on the stability of the region and the wider world?

Producer: Dan Tierney

Series producer: Amanda Hancox.