People of Irish descent form the largest single ethnic group in Boston, Massachusetts. Once a Puritan stronghold, Boston changed dramatically in the 19th century with the arrival of European immigrants. The Irish dominated the first wave of newcomers during this period, especially following the Great Irish Famine. Their arrival transformed Boston from an Anglo-Saxon, Protestant city into one that has become progressively more diverse. The Yankees hired Irish as workers and servants, but there was little social interaction. In the 1840s and 50s, the anti-Catholic, anti-immigrant Know-Nothing movement targeted Irish Catholics in Boston. In the 1860s, many Irish immigrants fought for the the Union in the American Civil War, and that display of patriotism helped to dispel some of the prejudice