Denunciation of the Soviet - Japanese Neutrality Pact

[RedRight]

Article Two: If one of the Contracting Parties becomes the object of hostilities by one or more third powers, the other Contracting Party shall remain neutral throughout the duration of the conflict.

Which can be understood as meaning that if one of the parties is the aggressor itself, the other may declare war on it.

Then a quote from the USSR's declaration of war: "join the war against Japanese aggression."

Have I logically linked these two passages? So did the Soviet Union not only have the moral right to join the war to shorten it, hand over Manchuria to China, etc., but also a legal justification?

[Art]

Which can be understood as meaning that if one of the parties is the aggressor itself, the other may declare war on it.

I don't think so. The article 3 said that the pact is automatically prolonged unless denounced by any side not later than 13 April 1945. Which the Soviet government did on 5 April 1945, about a weak before the formal deadline. Until that moment there was no question that the treaty somehow expired or lost its legal force.

[AriX]

Have I logically linked these two passages? So did the Soviet Union not only have the moral right to join the war to shorten it, hand over Manchuria to China, etc., but also a legal justification?

No, you get it in reverse.