You've got your Conventions mixed up. The Geneva Convention doesn't determine whether a war is a "just war" or not since the Geneva Conventions are about the protection of non combatants, wounded and POWs during wartime.

The Hague Conventions were/are about what you could call a "just war" since it states how wars should start between countries and how they should be conducted.

Opening of Hostilities (Hague III); October 18, 1907

http://avalon.law.yale.edu/20th_century/hague03.asp

Laws and Customs of War on Land (Hague IV); October 18, 1907

http://avalon.law.yale.edu/20th_century/hague04.asp

I've chosen the 1907 one because the war that most (weasel word I know) people seem to believe was a just war was the Second World War and so the 1907 Convention would be the one that would deal with WW2.

Both Poland and Germany were signatories to the Hague Convention

https://en.wikipedia.org/wiki/List_of_parties_to_the_Hague_Conventions_of_1899_and_1907

which would mean that Article 1 would apply between them

The Contracting Powers recognize that hostilities between themselves must not commence without previous and explicit warning, in the form either of a reasoned declaration of war or of an ultimatum with conditional declaration of war.

As far as I am aware Germany did not do this as they faked a Polish attack on German soil to make it look like Germany would be justified in defending itself. The Allies (France and the UK) then declared war on Germany for attacking Poland. That seems to be following the Hague Convention.

However, throughout the war we know that both sides didn't always follow the rules. Whether or not you consider that to remove or limit the justification is up to you.