Explanation: Leftists are... somewhat infamous for infighting amongst themselves, often to the detriment of their cause(s).
But during the US Civil War, a decent amount of unity was actually effected!
The Union had several literal Communists in its ranks, including the revolutionary German August Willich (who considered Marx too conservative), who served as a Major General during the war. Marx himself would write in support of the cause of the Union, and call Lincoln "the single-minded son of the working class". For its part, the Republican Party at this time was a broad coalition of farmers, businessmen, and labor radicals, whose primary uniting cause was anti-slavery. Lincoln himself would (briefly) make time to support labor rights at several points even during the brutal and ongoing Civil War.