Russia's aggression against Ukraine dates back to its 2014 annexation of Ukraine's southern peninsula, Crimea. 1 It wa.s Russia's brazen full-scale invasion of Ukraine on February 24, 2022,2 however, that captured global attention and put the crime of aggression - the resort to war in violation of the UN Charter; - in the spotlight. Iii recent months, model indictinents of Russiaii President Vladimir Putin, as well as Foreign Minister Scrgei Lavrov, Defence Minister Sergei Slioigii and others, have been widely publicized.5 A 2010 amendment to the Rome Statute of the International Criminal Court means that the ICC now has jurisdictioii over the crime of aggression when there is state consent.6 But Russia has not - and will not - provide such consent. If an aggression indictment is to be fornially issued, it will need to coine through a Ilewly created tribunal. Just a week after R.iissia's February 24 invasion, Chatham House convened a high-profile declaratioii to support the establishment of a tribunal to prosecute aggression.8 The following month, Ukrainian President Volodymyr Zelenskyy used his address to the UN Seciirity Council to call for the creation of an aggression tribunal.9 Since then, the Ukrainian government has beeii working with willing governments in Eiirope on plans to bring such a tribunal to life. Historically, powerful states have been reticent to let international criminal law encroach on decisions aboiit the use of force.11 So, tile plans currently beiiig developed may ultimately be blocked by tlie self-interest of states - the United Stiates among the forefront - in fear of the precedent an aggression tribunal would set. Indeed, this is the most likely outcome. But. as yet, the U.S. at least has not publicly registered any siicli dissent.12 Moreover, Russia's invasion seems to have pushed states out of busiiiess-as-usual positioning in all manner of ways. n Therefore, it remains at least a possibility that such a tribunal will be established.11 If it is, ariel especially if it issues indictments, this will have significant implications for the development of international law, regardless of whether those accused are ever arrested and brought to trial. Tile first implication, which fornis the bulk of the following ana.lysis. relates to the law on head of state immunity.16 The establishment of an aggression tribmial that enables the indictment of Putin or Lavrov while they remain iii their existing roles will constitute, iii Rebecca Ingber's terminology, an "interpretaition catalyst." 17 The tribunal's establishment will trigger legal interpretations by states on the topic of illimunity ratione personae.18 This will contribute to state practice and opinio juris. regardless of whether the tribunal pursues indictments or ever brings any accused into custody. If a case ever does proceed, the tribunal's decision on the immunity challenge tliat these defenda.nts would inevitably bring, woiild further contribute to our zinderstanding of this fraught area of interilatioiial law. Tlie secoiid iiiiplication relates to our linderstanding of the scope of liability for tlie crime of aggression. Aggression is understood to be a lea;(lership crime. But the charging decisions made by the aggression tribunal will add granularity to this understan(ling.21 There is a meaningful difference, for example, between charging Putin alone. or charging hi111 together witli 15 meinbers of his national security or military leadersliip. Finally. the temporal jurisdiction laid out in any aggression tribunal's constitutive document will influence, albeit indirectly. our understanding of the scope of the crime of aggression. A temporal jurisdiction clause that begins oil February 24, 2022 will prevent a tribunal from considering acts short of full-scale invasion. This does not mean, of course, that Russia's acts of aggression prior to February 24, 2022 cio not constitute aggression, however. it does set the bar very high for the future engagement of international criminal law with the crime of aggression. If a case on these grounds proceeds, the first jurisprudence on aggression since Nureinberg will be liniited to this textbook example. [ABSTRACT FROM AUTHOR]