At the UN World Summit in 2005, world leaders agreed an outcome document that formalised the Responsibility to Protect (R2P) as ‘an emerging international security and human rights norm’. When UN Secretary-General Kofi Annan announced that the world had taken ‘collective responsibility to protect populations from genocide, war crimes, ethnic cleansing and crimes against humanity’, it appeared that a new era in international cooperation had arrived. This article explores three stages of R2P development from the 1990s to the present: the events that led to the idea and implementation of so-called humanitarian intervention, including the words and actions of Prime Minister Tony Blair in relation to Kosovo in 1999; the disputes that shaped negotiations surrounding R2P, highlighting how political compromise is embodied in the Responsibility to Protect text as an inherent weakness; while the final section uses events in Syria between 2011 and 2014 to explore the conflicting political interests that render the legal dimension of R2P impotent despite the enthusiastic support of its advocates. The article concludes that unremitting mutual opposition between Russia and Western members of the Security Council over Syria –fuelled in turn by geo-strategic and national interests as well as humanitarian concern –means that agreement on R2P and military intervention on humanitarian grounds is as far away as it was at the time of Blair’s naively optimistic words in April 1999. The limits of R2P have been reached.