Maybe you should look into what leftists actually said about wars and take issue with them -- as it is, you are fighting against a phantom.
Theorists of imperialism, from liberal to Marxist, have generally sought to explain why capitalism as a mode of production needs imperialism, how capitalist imperialism differs from empires of the pre-capitalist age and Soviet imperialism, and so on. Such theorists (with the exception of principled -- often religious -- pacifists) have never held that all wars are unjust, all wars are about imperialism, all wars happen because of agitations of all capitalists, etc.
The US Civil War, for instance, is one of the few just wars that the United States government has ever fought, in which the right side, the side that Marx, et al. supported, won.
As for World War 1, much of communists' energy, in practice, went into fighting against social patriotism, social chauvinism, etc. which had many leftists, socialists included, support their respective capitalist-imperialist governments.* (In other words, they thought that ideology mattered.) That is the practice that most leftists today, having become liberals in effect, have abandoned, siding with the left wings of capitalist-imperialist governments of their own countries against peoples of such countries as Afghanistan, Iraq, and Iran.
* <http://www.marxists.de/war/lenin-war/ch1.htm> V.I. Lenin Socialism and War
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Social-chauvinism is advocacy of the idea of "defence of the fatherland" in the present war. Further, this idea logically leads to the abandonment of the class struggle during the war, to voting war credits, etc. Actually, the social- chauvinists are pursuing an anti-proletarian, bourgeois policy; for actually, they are championing not "defence of the fatherland" in the sense of fighting foreign oppression, but the "right" of one or other of the "great" powers to plunder colonies and to oppress other nations. The social-chauvinists repeat the bourgeois deception of the people that the war is being waged to protect the freedom and existence of nations, and thereby they go over to the side of the bourgeoisie against the proletariat. In the category of social-chauvinists are those who justify and embellish the governments and bourgeoisie of one of the belligerent groups of powers, as well as those who, like Kautsky, argue that the Socialists of all the belligerent powers have an equal right to "defend the fatherland". Social-chauvinism, being actually defence of the privileges, advantages, robbery and violence of one's "own" (or every) imperialist bourgeoisie, is the utter betrayal of all socialist convictions and of the decision of the Basle International Socialist Congress. -- Yoshie