This can be stretched out to a novel's worth of explanation, but here's a summary. As in all occupations, many crimes were committed against the population, however, they were mild compared to the crimes happening in Europe or East Asia. Still, the brutality of the occupation (which later came to include the US) caused a surge in Iranian nationalism. As for Reza Shah (the monarch), he did not necessarily allow the allies to occupy Iran. When the first regiment of Iran's armed forces retreated in the face of the overwhelming British and Soviet forces, it set off a chain reaction and much of the army capitulated. While Iran's military was modernizing, it was not yet equipped to take on the British and Soviet armies. Rather than send his remaining forces to an unnecessary death, Reza Shah stepped down in favor of his son. For many, the greatest "atrocity" of the occupation was the ousting of Reza Shah, an established leader who was efficiently modernizing and industrializing the nation.