UN was not founded at that time, wasn't it?

[v0031]

At this point during World War II, the United States and Japan were engaged in battle in the Pacific; Japan was also at war with China. Roosevelt received intelligence reports that Japanese military forces had used poisonous gas and other forms of what he called inhuman warfare, including biological agents, on innocent Chinese civilians, which violated the Geneva Convention of 1925.
Roosevelt warned that if Japan continued to use chemical warfare against China, the U.S. would consider such actions tantamount2 to a chemical or biological attack on America and the United Nations and respond with similar attacks. The president minced3 no words, stating that retaliation in kind and in full measure will be meted4 out. We shall be prepared to enforce complete retribution5. Upon Japan will rest the responsibility.

And what does upon here mean? Until?

[○∙◄☻¥tπ[╪AV┼cVê└]

That was a name for the allies during WW2.

[Donerail]

"Upon" is basically a synonym for has in that sentence - "upon Japan shall rest the responsibility" just means that Japan shall have the responsibility to stop using chemical weapons.

[🐒Gods of Prosperity🔱🐲💸]

"Upon" is basically a synonym for has in that sentence - "upon Japan shall rest the responsibility" just means that Japan shall have the responsibility to stop using chemical weapons.

It means that the guilt for a chemical or biological attack by the Allies against Japan would lay upon the Japanese, because they did it first. 

a morally disgusting idea, honestly.