Complex sentence: The principal reason for this was to secure the supply lines to the Soviet Union and to protect British oil installations in Abadan.Reza Pahlavi, Shah of Iran, was considered to be pro-German so he was deposed and replaced by his son, Mohammad Reza Pahlavi.
Split into simpler sentences: The principal reason for this was to secure the supply lines to the Soviet Union and to protect British oil installations in Abadan. The Shah of Iran Reza Pahlavi was considered to be pro-German so he was deposed and replaced by his son, Mohammad Reza Pahlavi.