There were three movements of North America’s most infamous racist group, the Ku Klux Klan.
The second wave ran from around 1915 to 1944, and one of their aims was to “preserve American culture” by stopping European immigration (which was happening at a higher rate because of the two World Wars) into both the United States and Canada.
In this period, the US version of the KKK was supremely anti-Irish and anti-Italian (amongst other nationalities), and anti-Catholic. The Canadian version was concerned with trying to stop immigration from Eastern Europe, in an attempt to preserve Canada’s “British culture”.
Because of these racist views, St Patrick’s Day was a target.