… the victory of socialism in one country does not at one stroke eliminate all wars in general. On the contrary, it presupposes wars. The development of capitalism proceeds extremely unevenly in different countries. It cannot be otherwise under commodity production. From this it follows irrefutably that socialism cannot achieve victory simultaneously in all countries. It will achieve victory first in one or several countries, while the others will for some time remain bourgeois or pre-bourgeois. This is bound to create not only friction, but a direct attempt on the part of the bourgeoisie of other countries to crush the socialist state’s victorious proletariat. In such cases, a war on our part would be a legitimate and just war. It would be a war for socialism, for the liberation of other nations from the bourgeoisie. Engels was perfectly right when, in his letter to Kautsky of September 12, 1882, he clearly stated that it was possible for already victorious socialism to wage “defensive wars”. What he had in mind was defense of the victorious proletariat against the bourgeoisie of other countries.
Only after we have overthrown, finally vanquished and expropriated the bourgeoisie of the whole world, and not merely in one country, will wars become impossible. And from a scientific point of view it would be utterly wrong—and utterly unrevolutionary—for us to evade or gloss over the most important things: crushing the resistance of the bourgeoisie—the most difficult task, and one demanding the greatest amount of fighting, in the transition to socialism. The “social” parsons and opportunists are always ready to build dreams of future peaceful socialism. But the very thing that distinguishes them from revolutionary Social-Democrats is that they refuse to think about and reflect on the fierce class struggle and class wars needed to achieve that beautiful future.
Extract from The Military Programme of the Proletarian Revolution, September 1916