From what you suggested in our first conversation you will need to explore the nature of International Law and its relationship with Politics. For this, a very good and useful place to start is: Martti Koskenniemi, ‘The Politics of International Law’, (1990) 1 EJIL 4-32 – follow the footnotes and see his various other publications (and book) on the nature of PIL. The International Law textbooks and case & materials books generally provide a discussion of the nature of IL near the beginning, so I suggest checking through the library and you will find plenty more material to get you started on this issue.
You will need to then research humanitarian intervention and R2P extensively, to identify and explore the conceptual and legal basis to each and related debate. There is extensive writing to draw upon. With your chosen case-study being on Syria you will also need to generate a very clear understanding of the arguments made by various countries at the UNSC and by academics on the merits and legal issues pertaining to an intervention in Syria. In addition to the journals, law blogs (such as EJIL: Talk and opinio juris), the UN website will provide you the official documents and draft resolutions on Syria.
A few other general sources you may find helpful:
Carty, A., International Law and the Spirit of Anti-Colonialism: Europe Fights Back, 74 Mod. L. Rev. 135 (2011).
Gerry Simpson, Great Powers and Outlaw States: Unequal Sovereigns in the International Legal Order (Cambridge, 2004)
Anne-Marie Slaughter, ‘International Relations, Principle Theories’, Max Planck Encyclopedia of Public International Law (2011)
Crawford and Koskenniemi (eds) Cambridge Companion to International Law (Cambridge University Press 2012)
Martti Koskenniemi, ‘The Lady Doth Protest too Much: Kosovo, and the Turn to Ethics in International Law’, 65 Modern Law Review (2002)
Anne Orford, Reading Humanitarian Intervention: Human Rights and the Use of Force in International Law (Cambridge University Press 2003)
Carsten Stahn, ‘Between law-breaking and law-making: Syria, Humanitarian Intervention and What the Law Ought to Be’ (2014) 19:1 Journal of Conflict and Security Law 25