Vikings vs Norsemen - who wins?

When did Vikings become more used than Norsemen?

The terms are de-facto synonyms, but I had a feeling (let's be generous and call that a hypothesis) that Norsemen was more commonly used in the past, but that at some time Vikings had shot to popularity and dominated since.

To test the hypothesis I turned to the Google Book N-gram Viewer. Here are the results.

As you can see Norsemen was as frequently used or more used than Vikings up until about 1919. From that point onwards Vikings grew the more common term and the gulf widened substantially from the 1940s onwards.

Why? I don't know, but perhaps there were som influential books or films published around 1919 and in the 40s? And that helped to popularise the term Vikings. This is another investigation, and a more complex one.