Marx and Engels supported German nationalism against French encroachment in 1859.

Evidence

Engels’s reaction to the outbreak of war in 1859 between Austria, Sardinia, and France, and to the German nationalist movement that had attended the prelude of war, was to publish a pamphlet entitled Po und Rhein. … In his pamphlet, Engels argued that though the Po river boundary was of no military significance to Germany (in these terms, Engels added, France’s claims were more defensible), politically, the French encroachment on the Po was the beginning of an attempt to recapture the Rhine frontier. On this basis, Engels insisted, the Po was where Germany must make its stand with Austria. The pamphlet was published in Germany, anonymously, on the advice of Marx so as to give the impression that the author was probably a Prussian general—a stratagem that was apparently quite successful, to Marx’s great satisfaction. — Cedric J. Robinson, Black Marxism_ The Making of the Black…, 329