There is one, and only one, kind of real internationalism, and that is—working whole-heartedly for the development of the revolutionary movement and the revolutionary struggle in one’s own country, and supporting (by propaganda, sympathy, and material aid) this struggle, this, and only this, line, in every country without exception.
1). The social-chauvinists, i.e., socialists in word and chauvinists in deed. People who support “defence of the fatherland” in an imperialist war (and above all in the present imperialist war).
These people are our class enemies. They have gone over to the bourgeoisie.
What is Social Chauvinism? Social-chauvinism is adherence to the idea of “defending the fatherland” in the present war. From this idea follows repudiation of the class struggle in war time, voting for military appropriations, etc. In practice, the social chauvinists conduct an anti-proletarian bourgeois policy, because in practice they insist not on the “defense of the fatherland” in the sense of fighting against the oppression of a foreign nation, but upon the “right” of one or the other of the “great” nations to rob the colonies and oppress other peoples. The social-chauvinists follow the bourgeoisie in deceiving the people by saying that the war is conducted for the defense of the freedom and the existence of the nations; thus they put themselves on the side of the bourgeoisie against the proletariat. To the social-democrats belong those who justify and idealize the government and the bourgeoisie of one of the belligerent groups of nations, as well as those who, like Kautsky, recognize the equal right of the Socialists of all belligerent nations to “defend the fatherland.” Social-chauvinism, being in practice a defense of the privileges, prerogatives, robberies and violence of “one’s own” (or any other) imperialist bourgeoisie, is a total betrayal of all Socialist conviction and a violation of the decisions of the International Socialist Congress in Basle (1912, Ed.) ... Turn Imperialist War Into Civil War
Aggressive or fanatical patriotism, particularly during time of war, in support of one's own nation or other nation(s). During WWI, nearly every political party took a social-chauvinist stand; with few exceptions. Most Socialists gave up their beliefs in favour of "defense of the fatherland," and turned to social-chauvinism; most notably the German Social Democratic Party. Two outstanding examples of Communists who fought against social-chauvinism during WWI were Rosa Luxemburg and Karl Liebknecht. They stressed that the only violence that should be used is the violence necessary to overthrow one's own government. They agitated tirelessly in their nation to show that common social relations united workers across any national boundaries and that the only blood the proletariat should shed is the blood to gain their freedom.
See:
Lenin: Lessons of the Revolution
Lenin: Opportunism, and the Collapse of the Second International