American neutrality might end in fighting without a formal declaration

Would the United States enter the Second World War if Britain faced defeat?

A declaration made only for Britain was unlikely. Worsening conflict with Germany and Italy could still bring the United States into the war.

A formal declaration made specifically to save Britain is different from the broader path toward American belligerence. The United States would be unlikely to enter on that narrow rationale alone. At the same time, relations with Nazi Germany and Fascist Italy were already creating other routes to armed conflict. Britain facing defeat could therefore accelerate intervention even if the legal and political trigger came from a different confrontation.