Dressed in black, 21 million pilgrims from around the world massed in the Iraqi city of Karbala on Saturday for the Arbaeen commemoration, against the backdrop of a political crisis. Arbaeen marks the end of the 40-day mourning period for the 7th-century killing of Imam Hussein by the forces of the Caliph Yazid — a formative event in Shia Islam. The annual festival sees men and women from across Iraq and beyond travel to Karbala, where Imam Hussein and his brother Abbas are buried, for one of the world’s largest religious gatherings. After two years marked by the Covid pandemic and border res…