Two eras of war

Marx and Engels did not abstain on the question of war. They took definite sides on particular wars using the criteria of victory or defeat for which of the belligerent camps most represented historical progress. That approach was appropriate to the era of ascending capitalism when the socialist revolution was not yet a historic possibility. In the period from the American War of Independence of 1776 until the Paris Commune of 1871, the capitalist class could still play a historically progressive role. The wars led by the bourgeoisie overturned outmoded feudal relations, clearing the way for the more rapid development of capitalism and of its unique product, the modern working class, the embodiment of socialised labour.
The best example of a war of this type was revolutionary war waged by the revolutionary French Republic at the end of the 18th century. The spread of the French Revolution challenged feudalism throughout Europe. Had the revolution not spread to the rest of Europe, it would have been strangled by aristocratic counter-revolution from abroad. The radical left of the time, the Jacobins, were aggressive proponents of revolutionary warfare.
Other progressive, bourgeois wars of this period included the wars of German and Italian unification, which ended the division of those countries into feudal mini-states and created unified national states and markets.
Another type of progressive war of that period was civil war to end the racial slavery. The Haitian Revolution at the beginning of the 19th century resulted in a long period of brutal warfare for human freedom from racial slavery. Similarly, the American Civil War between the Northern industrial capitalism, in alliance with the free blacks of the North and the slaves of the South, against the Southern Confederacy of capitalist plantation slave owners was a war for human liberation as well as for capitalist progress. It was one of the last acts, anywhere in the world, of the bourgeoisie as a progressive class.
To dismiss these events as limited because they were bourgeois is ahistorical. Compared to past conditions they were progressive historical developments. They created industrial capitalism whose mechanised means of production and proletarian labouring class are the preconditions for socialism. Marx and Engels decided their positions on the wars of that era based on which side’s victory was a victory for historical progress, and which side represented the future interests of the working class.