There are two kinds of international law in issue here:
- the law that governs nations, which is of some long standing and fairly well developed (and is administered by the International Court of Justice (ICJ) between states parties to treaties), and
- the law that governs individuals, which is novel and very incomplete (and is administered by the International Criminal Court (ICC) when it tries individuals on charges under the Rome Statute).
Individual countries also incorporate "international" crimes like genocide and war crimes in their own criminal law, and can try individuals over whom they have jurisdiction.
Hypothetically, Iraq could take the US before the ICJ for violating international agreements to which it is a party, as Nicaragua did in respect of all the acts of aggression committed against it by the US.
To hold Bush, or anyone,
personally criminally liable for acts of aggression would require either that there be a "crime of aggression" defined in US law, and that he be tried in a US court, or that there be a "crime of aggression" defined in international law and that the ICC acquire jurisdiction over him somehow.
There is no crime of aggression in US law (although US law does incorporate the crime of genocide, war crimes and crimes against humanity, the other internationally recognized crimes).
And there is also no crime of aggression in international law, because that crime has simply never been defined:
The Rome Statute of the International Criminal Court (which the US has of course not ratified) provides:
http://www.icrc.org/IHL.nsf/52d68d14de6160e0c12563da005fdb1b/fb2c5995d7cbf846412566900039e535?OpenDocumentArticle 5
Crimes within the jurisdiction of the Court
1. The jurisdiction of the Court shall be limited to the most serious crimes of concern to the international community as a whole. The Court has jurisdiction in accordance with this Statute with respect to the following crimes:
(a) The crime of genocide;
(b) Crimes against humanity;
(c) War crimes;
(d) The crime of aggression.
2. The Court shall exercise jurisdiction over the crime of aggression once a provision is adopted in accordance with articles 121 and 123 defining the crime and setting out the conditions under which the Court shall exercise jurisdiction with respect to this crime. Such a provision shall be consistent with the relevant provisions of the Charter of the United Nations.
And that has not yet been done. The Rome Statute defines (a), (b) and (c) at great length, but says nothing more about aggression.
So
the United States could be brought before the International Court of Justice for committing acts of aggression contrary to specific provisions in international law (treaties), but
George W. Bush could not be prosecuted for such acts anywhere.
Now, the definition of "War Crimes" in Article 8 does include these:
(b) Other serious violations of the laws and customs applicable in international armed conflict, within the established framework of international law, namely, any of the following acts:
(i) Intentionally directing attacks against the civilian population as such or against individual civilians not taking direct part in hostilities;
(ii) Intentionally directing attacks against civilian objects, that is, objects which are not military objectives;
(iii) Intentionally directing attacks against personnel, installations, material, units or vehicles involved in a humanitarian assistance or peacekeeping mission in accordance with the Charter of the United Nations, as long as they are entitled to the protection given to civilians or civilian objects under the international law of armed conflict;
(iv) Intentionally launching an attack in the knowledge that such attack will cause incidental loss of life or injury to civilians or damage to civilian objects or widespread, long-term and severe damage to the natural environment which would be clearly excessive in relation to the concrete and direct overall military advantage anticipated;
(v) Attacking or bombarding, by whatever means, towns, villages, dwellings or buildings which are undefended and which are not military objectives; ...
... and others, but again, the ICC would not have jurisdiction over Bush unless somebody forcibly brought him before that court (and Iraq, which I assume has not ratified the Rome Statute, accepted the ICC's jurisdiction).
The US's own law on war crimes (under which a US president could be charged if he had committed one of the offences in US law) may be read here:
http://caselaw.lp.findlaw.com/casecode/uscodes/18/parts/i/chapters/118/sections/section_2441.html.