Archives about Catholic ecclesiastical affairs, leaders and prominent figures may not seem to be the most obvious place to look for evidence of Copperhead activity during the Civil War. But consider, for example, the James McMaster papers in the Notre Dame archives, which (like other items in the archives) are not only available to researchers on site but available in summary form in the Univerrsity of Notre Dame Archives’ online Calendar.
Consider Stephen E. Towne, in his 2015 account of the Northern struggle against conspiracies on the home front.
Towne has extensively researched many archives, mainly showing the evidence accumulated by government and military surveillance on the “Copperhead” movement (Northerners actively opposing the Northern war effort). Based on these documents, Towne disagrees with, say, Frank Klement, who thought the Copperheads were lawful political opponents of the Lincoln administration who were demonized as subversives in order to whip up support for the war effort and the Republicans. Towne is one historian who thinks there were actually illegal conspiracies by Copperheads to obstruct the war effort or even work with the Confederates.
A lot of Towne’s sources come from government documents – he complains that existing archives have “[v]ery few records” of Copperhead activity. One exception Towne cites: the papers of James McMaster, editor of the the New York Freeman’s Journal and Catholic Register and a prominent Coperhead. An influential Catholic polemicist in his above-ground life as an editor, his papers, lodged at Notre Dame, indicate that he was hip-deep in subversion. Towne cites some of the highlights: McMaster’s association with Confederate agent Emile Longuemare, his correspondence with Confederate commissioners in Canada, and his activities in Copperhead secret societies – the Organization of American Knights and the Sons of Liberty.
I fleshed out some of these details in an article, “The Radicalization of James McMaster,” basing my findings of subversion on information in McMaster’s papers. McMaster corresponded with Longuemare about secret Copperhead societies. He kept in touch with Jacob Thompson and Clement C. Clay, Confederate commissioners in Canada. A former Confederate operative, after McMaster’s death, even claims that McMaster was, at least initially, willing to take part in an abortive Confederate plot to burn New York City, where McMaster himself worked.
These were perhaps the most dramatic examples of McMaster’s wartime correspondence with Confederates and Confederate sympathizers, as documented in my article. Because McMaster kindly preserved his records, rather than burning them like a sensible person, we have a more fully fleshed-out picture of Copperhead activity.
The Notre Dame Archives, and its online Calendar, have a good deal of other Civil War-related information involving the Catholic Church – ecclesiastical records and correspondence and other essential information about the intersection of the spiritual with the civil strife.
So I will nominate these Catholic records as among the valuable sources of information about the war.
 Stephen E. Towne, Surveillance and Spies in the Civil War: Exposing Confederate Conspiracies in America’s Heartland (Athens: Ohio University Press, 2015), 310.
 Max Longley, “The Radicalization of James McMaster: The ‘Puritan’ North as an Enemy of Peace, the constitution, and the Catholic Church,” U. S. Catholic Historian, Vol. 36, No. 4 (Fall 2018), 45-47.