What is nativism and why did it happen?

Nativism is an anti-catholic and anti-immigrant movement.
It became really big because in 1834 nativists had burned down a convent in Massachusetts, not 10 years later some rioters then burned down two churches in Phili. There was a long period in which many immigrants came to america which caused many natives of America to feel less valued or taken over and they did not like this.
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