• The First Republican Army: The Army of Virginia and the Radicalization of the Civil War by John H. Matsui (Univ of Va Pr, 2016).
John Pope gained early-war notoriety for targeting Missouri civilians in retaliation for guerrilla attacks on infrastructure. But he's better known for the bombastic manner by which he brought this harsher brand of western frontier warfare east to Virginia, where he assumed command of the Army of Virginia and quickly drew the ire of Robert E. Lee. In his book The First Republican Army, author John Matsui argues that ideological differences between the Army of Virginia and the Army of the Potomac ran much deeper than the politics, mindset, and leadership style of the man at the top.
From the description: "If the Army of the Potomac (the major Union force in Virginia) was dominated by generals who concurred with the ideology of the Democratic Party, the Army of Virginia (though likewise a Union force) was its political opposite, from its senior generals to the common soldiers. The majority of officers and soldiers in the Army of Virginia saw slavery and pro-Confederate civilians as crucial components of the rebel war effort and blamed them for prolonging the war. The frustrating occupation experiences of the Army of Virginia radicalized them further, making them a vanguard against Southern rebellion and slavery within the Union army as a whole and paving the way for Abraham Lincoln to issue the Emancipation Proclamation."
It looks like the author assembled a pretty good-sized study group. "Drawing on archival sources from twenty-five generals and 250 volunteer officers and enlisted men, John Matsui offers the first major study to examine the ways in which individual politics were as important as military considerations to battlefield outcomes and how the experience of war could alter soldiers’ political views." If I recall correctly, Joseph Glatthaar's cohort numbered 600 for his celebrated quantitative Army of Northern Virginia study.