I was doing a bit of history hunting and it turns out if you want to really keep the time frame for Ranma 1/2, or even bring it a little bit forward, knowing the History during the new Shōwa era from 1926-1989 might not hurt.
Here is the link to the Wikipedia Entry. [url=http://en.wikipedia.org/wiki/Shōwa_period]Shōwa period[/url]
Link failure here it is bare: http://en.wikipedia.org/wiki/Shōwa_period
If anyone else has some good links to details on different areas of Japan during this Era it can only help.