The following quote, found over at the Testify! blog, adds to the evidence that starting a war without support from other world leaders is not only wrong, but quite likely criminal.
“We must make clear to the Germans that the wrong for which
their fallen leaders are on trial is not that they lost the
war, but that they started it. And we must not allow
ourselves to be drawn into a trial of the causes of the war,
for our position is that no grievances or policies will
justify resort to aggressive war. It is utterly renounced
and condemned as an instrument of policy.”
U.S. Supreme Court Justice Robert L. Jackson
Chief U.S. Prosecutor
at the Nuremberg Tribunals
August 12, 1945
Emphasis added. Additional note: the above is speaking to nations acting in a singular manner, or perhaps w/one or two allies. Coalitions of a majority of countries of the world are often a quite different matter.
The full statement is available.