Both more specifically and more generally: did Tony Blair, the rest of
the British government, a large proportion of Parliament, a swathe of
the civil and diplomatic services, the monarchy and the military
commit a war crime through their involvement in the planning,
preparation, initiation or execution of the 2003 invasion of Iraq?
IANAL and I'm
struggling a bit with some of this stuff but
I think that, strictly speaking, they did not though only because waging
a war of aggression or a war in violation of international treaties is
not a war crime but
rather, to use the terminology of the Nuremberg trials as it has
entered current international criminal law, a crime against peace.
Rudolph Hess's conviction is a relevant case;
he was found guilty of participating
in waging aggressive war but not guilty of war crimes
(or crimes against humanity).
The distinction is a bit pedantic but it does have unfortunate
consequences for the legal situation in British law and for
jurisdiction in international law.