From Axis and allies 1940 europe second edition rule book p.10
The United States may not declare war on any Axis
power unless an Axis power either declares war on it first
or captures London or any territory in North America,
after which it may declare war on any or all Axis powers
on its following turn.
The capture of London is not included in the notes of the tripleA engine, this can have a major impact on strategy if Germany wants to do sea lion.