Explanation: Leftists are… somewhat infamous for infighting amongst themselves, often to the detriment of their cause(s).

But during the US Civil War, fought against literal chattel slavery of Black folk against an ideologically pro-slavery secession movement, 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.