Did japan plan to attack Baja California Mexico?
Not really; there is an urban legend stating that there was a covert submarine refueling base near Magdalena Bay in Baja California, but it has never been proved.
At the time (1942), infrastructure was insignificant, meaning Baja California didn't have the requirements to establish a foothold on the Americas to attack the United States or conquer Mexico.
The Baja California peninsula (meaning Lower California) is part of Mexico and has always been since Mexico's independence in 1821. Before becoming a state in 1953, Baja was known as the North Territory of Baja California. Right now it is conformed by two federal states: Baja California (Capital: Mexicali) and Baja California Sur (Capital: La Paz). Baja California is both the northernmost and westernmost state of Mexico.
It's in Mexico. If you look at a map of the western coast, Baja California is the long peninsula that sticks out south of California. In Spanish, Baja California means "Lower California"- this is because California was once part of Mexico. The US took it from Mexico during a war in the mid-1800s; prior to this, California was known as Alta California ("Upper California").