This thesis, selected as one of the best LL.M submissions at CIDS/MIDS, explores a critical and underexamined frontier in international commercial arbitration: the growing presence of States and State-owned enterprises (SOEs) in a traditionally private dispute-resolution forum. As public actors increasingly engage in cross-border contracts, they bring with them non-waivable duties, mandatory norms, and public-interest concerns that challenge the conventional paradigm of party autonomy, confidentiality, and limited judicial oversight. Through a comparative analysis of France, Germany, the United Kingdom, and Brazil, the thesis demonstrates that while each jurisdiction protects public interest differently - via narrowed arbitrability, mandatory regimes, or constitutional constraints - they converge on the need for tribunals to engage transparently with public-law norms. The proposed three-step method-identification, justification, and proportionality-offers a principled framework for arbitrators to write awards that are review-resistant, publicly intelligible, and legally enforceable. Ultimately, the thesis argues that public interest is not a threat to arbitration's legitimacy, but a call for better reasoning. When tribunals "show their work," they preserve efficiency while reinforcing trust in arbitration's capacity to handle disputes with public consequences.