This Article explores three theories of humanitarian intervention that appear in, or are inspired by, the writings of Hugo Grotius. One theory asserts that natural law authorizes all states to punish violations of the law of nations, irrespective of where or against whom the violations occur, to preserve the integrity of international law. A second theory, which also appears in Grotius’s writings, proposes that states may intervene as temporary legal guardians for peoples who have suffered intolerable cruelties at the hands of their own state. Each of these theories has fallen out of fashion today based on skepticism about their natural law underpinnings and concerns about how they have facilitated Western colonialism. As an alternative, this Article outlines a third theory that builds upon Grotius’s account of humanitarian intervention as a fiduciary relationship, while updating Grotius’s account for the twenty-first century. According to this new fiduciary theory, when states intervene to protect human rights abroad they exercise an oppressed people’s right of self-defense on their behalf and may use force solely for the people’s benefit. As fiduciaries, intervening states bear obligations to consult with and honor the preferences of the people they seek to protect, and they must respect international human rights governing the use of force within the affected state. By clarifying the respective responsibilities of the Security Council and individual states for humanitarian intervention, the fiduciary theory also lends greater coherency to the international community’s “responsibility to protect” human rights.