Megan works in public international law, its theory and history.

Her historical work has examined the development of the international legal order over the nineteenth and twentieth centuries, particularly foundational areas like the law of statehood, treaties and diplomatic relations. She is interested in methodological dimensions of the interaction between law and history: how each approaches texts and material evidence; how different scales (national, imperial, global) and strands (intellectual, social, cultural) of historical scholarship might engage differently with law; and the possibilities for more fruitful collaboration between these fields in future. Work on contemporary international law has explored the workings of international institutions, with a particular focus on transparency; public law and constitutional values; and the use of languages of public law in global governance.

A study in the history of treaty-making—'The Survival of the Secret Treaty: Publicity, Secrecy, and Legality in the International Order'—won the Francis Deák prize (2017) for the leading article by a younger author in the American Journal of International Law. Other recent publications include an article on 'The League of Nations, Ethiopia, and the Making of States' (Humanity (Spring 2020)), and an edited collection (with Annabel Brett and Martti Koskenniemi), History, Politics, Law: Thinking through the State (CUP, 2021).  

Megan is now working on a book tracing secrecy and publicity in the international legal order (under contract with OUP). Her main future project is on the conceptualization of peace and peace-making, particularly the faultline between scholarship on imperial governance and on peace agreements between European powers (terrain sketched in 'The Boundaries of Peace-Making: British Imperial Encounters c.1700–1900', in Weller, Retter and Varga (eds), Peace Agreements and International Law (CUP, 2021)). Other current work includes a chapter on legal thought in the early League of Nations (in collaboration with the project Laying the Foundations: The League of Nations and International Law, 1919–45, at the University of Copenhagen); and research into the absence of prosecutions of Italian forces for atrocities committed in Ethiopia.