rule with a rod of iron
To rule, govern, or control a group or population with complete power over all aspects of life, work, etc. He rules with a rod of iron, and moves swiftly to gain control over any entity that is not already in his grasp. She has ruled this company with a rod of iron for three decades, and it's going to be difficult for her to let go of control.
rule someone or something with a rod of iron
control or govern someone or something very strictly or harshly. This expression comes from Psalm 2:9: ‘Thou shalt break them with a rod of iron; thou shalt dash them in pieces like a potter's vessel’.