If a German under Wilhelm or a Frenchman under Clemenceau says, “It is my right and duty as a socialist to defend my country if it is invaded by an enemy”, he argues not like a socialist, not like an internationalist, not like a revolutionary proletarian, but like a petty-bourgeois nationalist. Because this argument ignores the revolutionary class struggle of the workers against capital, it ignores the appraisal of the war as a whole from the point of view of the world bourgeoisie and the world proletariat, that is, it ignores internationalism, and all that remains is miserable and narrow-minded nationalism. My country is being wronged, that is all I care about—that is what this argument amounts to, and that is where its petty-bourgeois, nationalist narrow-mindedness lies. […] The Frenchman, German or Italian who says: “Socialism is opposed to violence against nations, therefore I defend myself when my country is invaded”, betrays socialism and internationalism, because such a man sees only his own “country”, he puts “his own” ... bourgeoisie above everything else and does not give a thought to the international connections which make the war an imperialist war and his bourgeoisie a link in the chain of imperialist plunder. […]
The socialist, the revolutionary proletarian, the internalionalist, argues differently. He says: “The character of the war (whether it is reactionary or revolutionary) does not depend on who the attacker was, or in whose country the ‘enemy’ is stationed; it depends on what class is waging the war, and on what politics this war is a continuation of. If the war is a reactionary, imperialist war, that is, if it is being waged by two world groups of the imperialist, rapacious, predatory, reactionary bourgeoisie, then every bourgeoisie (even of the smallest country) becomes a participant in the plunder, and my duty as a representative of the revolutionary proletariat is to prepare for the world proletarian revolution as the only escape from the horrors of a world slaughter. I must argue, not from the point of view of ‘my’ country (for that is the argument of a wretched, stupid, petty-bourgeois nationalist who does not realise that he is only a plaything in the hands of the imperialist bourgeoisie), but from the point of view of my share in the preparation, in the propaganda, and in the acceleration of the world proletarian revolution.”
That is what internationalism means, and that is the duty of the internationalist, the revolutionary worker, the genuine socialist.
Lenin, The Proletarian Revolution and the Renegade Kautsky (1918)