Abstract
This paper examines the legal status of humanitarian intervention in the aftermath of the abortive military operation in Syria. In tracing the history of the doctrine up to the beginning of the Syria crisis, it asserts that the negative reaction to the abortive use of force has reinforced the unlawfulness of humanitarian intervention. In appraising in detail the legal positions of the UK, the USA, France and other states, it analyses the interplay of constitutional law and international law in the Syria crisis with reference to the role of consultations of national parliaments. It asserts that the Syrian precedent illustrates the difficulty of identifying the opinio iuris of a state in which the input of international law through legal advice and public debate affects the output of that state through the expression of its legal position on doctrinal questions. By scrutinising the approach of the British government in consulting the House of Commons in relation to the continuing operations against Islamic State in Iraq, it argues that the National Security Strategy review in 2015 should address the concerns expressed by parliamentary committees regarding consultation of Parliament, the provision of legal advice and the lawfulness of humanitarian intervention.