The fear of foreigners during the 1920s was part of the return to

The return to normalcy

Explanation:

The "return to normalcy" was a concept that was popularized by Warren G. Harding's 1920 presidential campaign. This was his slogan, and it summarized  many of the policies that Harding wanted to pursue. "Normalcy" was a term that described the mentality that existed in the United States before the war. This meant a return to peace and a conservative approach to life. However, it also implied a nationalist outlook that led to fear of foreigners.

"the royal proclamation of 1763"