The invasion of Iraq in 2003 sparked a competition between two major Middle Eastern powers, the US and Iran, for influence in the former Ba’athist dictatorship. For the US, influencing Iraq would reinforce its position, strengthen Washington’s security in the international system by turning a former enemy into an ally, and provide the US with the ability to engage Iran, its chief rival more effectively by positioning itself right on Tehran’s border. It would also help secure its border by ensuring that Baghdad would never rise to threaten Tehran’s national security in the future and decrease American power in the wider region while enhancing its own capabilities in the Middle East. To interrogate this competition, the thesis investigates the strategic competition between the US and Iran from 2003-2016 through the lens of structural realism (and its offensive and defensive sub-variants) and examines the tactics, actors, and policies adopted by each side in their quest to gain an advantage over the other. It finds, ultimately, that Iran proved to be more successful than the US in acquiring a position of relative strength in Iraq.