The refusal of Archbishop Desmond Tutu to attend a conference along with former Prime Minister Tony Blair made headlines -Why I had no choice but to spurn Tony Blair - The Guardian 2nd September. Tutu went further and argued that Blair and Bush ought to be tried by the International Criminal Court - The Telegraph 2nd September - Archbishop Tutu - Try Blair for War Crimes. The actions of Blair and Bush in relation to Iraq will always be controversial and debate continues as to whether the invasion of Iraq (a sovereign State) in 2003 was lawful. However, this post is not concerned with either the legality or the "morality" of the war but only with the question of the ICC's jurisdiction since trials are only possible where the court has jurisdiction.
The United States of America has not accepted the court's jurisdiction and also Iraq is not a States Party. The United Kingdom became a States Party on 4th October 2001. The court only has the jurisdiction granted to it by the Rome Statute on the International Criminal Court - see ICC website - jurisdiction and Court's Statute.
The court's jurisdiction:
The Court does not have universal jurisdiction.* The Court may only exercise jurisdiction if:
The accused is a national of a State Party or a State otherwise accepting the jurisdiction of the Court;
The crime took place on the territory of a State Party or a State otherwise accepting the jurisdiction of the Court; or
The United Nations Security Council has referred the situation to the Prosecutor, irrespective of the nationality of the accused or the location of the crime.
It would seem that only the first of those points applies to Blair. Bush would seem to be completely safe from the tentacles of the court.
Furthermore, the ICC has no power to act in relation to events before 1st July 2002. Note also the principle of "complementarity."
The Court may exercise jurisdiction over the crime of aggression after 1 January 2017 provided that at least thirty States Parties have ratified the amendments and the ICC Assembly of States Parties has agreed to activate that jurisdiction. The 30th State Party to ratify was Palestine on 26th June 2016 - see ICC announcement HERE. The requirement for the ICC Assembly of States Parties to agree to activate the jurisdiction has not yet been achieved - see the article by Dapo Akande at EJIL: Talk! 26th June 2017
When the ICC jurisdiction relating to aggression commences it will not be applied retrospectively. This appears to rule out jurisdiction relating to any events before 26th June 2016 - the date of the 30th ratification.