US Declarations of War
The US may only declare war in the following cases:
• Any major power declares war on the US.
• Japan declares war on any non-Vichy French country.
• If Japan makes more than one attack in China per season,
the US may declare war on Japan. Place the “Japan China
Attack” marker in the Season space of the TD as a reminder
that Japan has attacked China that turn.
• In Winter 1943 or later, the US may declare war on Germany.
There's a few more, but the designer seems to subscribe to the "we wouldn't have gone to war with Germany if they weren't stupid and DoW the US first" school of thought. The two ones you ignore seem to be the most likely: Japan declaring war on any non-Vichy country and Winter 1943 or later.