They stood on the sidelines during WWII hoping things went badly for the allies, and us, having negotiated with Germany over our southwestern territory.
I’ve been saying we should war with Mexico for years. They owe us about 30 years of serious reparations, and assigning Pemex profits is a good start.
Are you, perhaps, confusing WW II with WW I and the infamous Zimmerman Telegram?
The German Foreign minister did indeed offer Mexico a free hand in what is now the SW United States if they would enter WW I on Germany's side.
Mexico, however, refused and the contents of the telegram -- which had been intercepted and decoded by British Intelligence -- were passed to the USA. The Zimmerman Telegram subsequently became a part of America's casus belli for entering WW I.
I'm not aware of Mexico negotiating nor Germany proposing a similar arrangement during WW II.