Great open letter from Marx (on behalf of the International Working Men's Association) to Lincoln in 1864,
We congratulate the American people upon your re-election by a large majority. If resistance to the Slave Power was the reserved watchword of your first election, the triumphant war cry of your re-election is Death to Slavery.
From the commencement of the titanic American strife the workingmen of Europe felt instinctively that the star-spangled banner carried the destiny of their class. The contest for the territories which opened the dire epopee, was it not to decide whether the virgin soil of immense tracts should be wedded to the labor of the emigrant or prostituted by the tramp of the slave driver?
and here's more by Marx on the Civil War.