Two million Muslims from across the globe are converging on Mecca in Saudi Arabia for the hajj pilgrimage, a religious duty and for some pilgrims the journey of a lifetime.

This year sees the return of pilgrims from Shiite Iran, a regional rival to Sunni powerhouse Saudi Arabia, and comes during a time when the Gulf is mired in political crisis. Indonesia is the world's most populous Muslim nation, and also accounts for the largest number of pilgrims for the hajj. 

Iranians are back after not attending in 2016 following a deadly Mecca stampede the previous year that killed nearly 2,300 pilgrims. It was the worst catastrophe in the history of the hajj, with 464 people from the Islamic republic among the dead. Following the disaster, Tehran railed against Saudi Arabia's organization of the pilgrimage. The two countries then severed diplomatic relations in January 2016 after the Saudi embassy in Tehran was ransacked by a crowd protesting against the execution in the kingdom of a Shiite religious dignitary.

The hajj is one of the five pillars of the Islamic faith, which every Muslim is required to complete at least once in a lifetime if he or she has the means to do so.