The adoption of the UN Charter has totally reshaped the legal framework on the use of force. It marked an “extraordinary departure” from existing customary international law (Gray, International law and the use of force. Oxford University Press, 2018, p. 6), since the Charter outlaws war as state policy and places collective security at the center of its foundation (Franck, Recourse to force, state action against threats and armed attacks. Cambridge University Press, 2002, p. 2). State practice and doctrines developed before WWII, like PNA, are subject to the conditions set by Arts 2(4) and 51. Generally, the Charter has reversed the broad connotation following states right to self-defense, since it is formulated as an exception to the prohibition of the use of force rule (Parry, Canadian Yearbook of International Law 26:253–272, 1988, p. 254). In this regard, invocation of the PNA doctrine by states has been scrutinized, pointing at the doctrine’s incompatibility with the prohibition of the use of force, or its potential justification under the scope of Art. 51’s right to self-defense. Its interpretation falls under the scheme created by Arts 2(4) and 51. The debate on the doctrine’s legality has been extensive and forms part of a wider discussion regarding the interpretation of Arts 2(4) and 51 in the post WWII era and the legal regulation of the use of force. This chapter analyzes whether there is room under Art. 2(4) for the use of force to protect nationals abroad, according to its predominant readings and interpretations, and further if it falls under the right to self-defense spectrum, qualifying the criteria set by the UN Charter. In addition, it examines the correlation between forcible PNA and the doctrines of humanitarian intervention and responsibility to protect (R2P). Following this analysis, it explores if states bear a duty to protect their nationals abroad, explaining how this may contravene the concepts of sovereignty and territoriality.