Organized along chronological lines, the work examines whether, or to what extent, the US position has evolved over time. The book explores how the definition of the crime can impact upon the US, notwithstanding its failure to ratify the Rome Statute. It also shows that the US practice and opinio iuris about the law applicable to the use of force might influence, as it has done in the past, the law itself. The work will be a valuable guide for students, academics and professionals with an interest in International Criminal Law.