Earlier this month, while editing a document for a client, I came across the following phrase:
World War 2
I immediately flagged it for the client, recommending that they change it to “World War II”. Here’s why.
When we need to indicate that a proper noun is part of a series, we use roman numerals to do so. That’s why we have Super Bowl XLIX, Charles Emerson Winchester III, and Queen Elizabeth II. Hence, World War II.
On a related note, you can also write it as Second World War.