I don't see why there's this assumption that Marx would side with the Mensheviks over the Bolsheviks. Marx would have recognised, just as Luxemburg, Debs, Gramsci and Lenin did, that the first world war was a war between imperialist aggressors and should be opposed (whereas the Menshiviks and other SocDem movements throughout Europe capitulated to petty nationalism).
First of all, I didn't say that Marx would have sided with the Mensheviks in the party split. I said that *if he did,* Lenin would ignore or dismiss his views. Second, I was talking about the split which took place years before the War. In the 1900's there was no disagreement between Bolsheviks and Mensheviks on whether socialists should oppose imperialist wars. Their disagreements were largely about organizational questions. Most western European social democrats who knew about the split were puzzled by it and didn't think there were sufficient grounds for it. Bebel told Maxim Litvinov, "But you are children!"
http://books.google.com/books?id=dN5V8WX5WP0C&pg=PA266 (That might, or might not, have been Marx's position.) Third, it is not true that the Mensheviks came out in favor of the war in 1914. The Mensheviks and Bolsheviks in the Duma walked out of the session and refused to vote for credits--and then issued a joint statement attacking the “false patriotism under which the ruling classes wage their predatory policy." Now one can argue that the statement was not unequivocally anti-war enough, because it also stated "that the proletariat would defend the cultural treasures of the people against all attacks, no matter where they came from, whether from within or from without."
http://www.marxists.org/archive/cliff/works/1976/lenin2/ch02.htm But equivocal or not, it was agreed to by Bolsheviks and Mensheviks alike, and was more anti-war than the position of any other social democratic party in Europe. (Plekhanov, who was enthusiastic about the war, was as isolated from the Menshviks as from the Bolsheviks.) The Mensheviks did not become clearly "defensist" until after the February Revolution.
Marx and Engels during their lives were not neutral in capitalist wars (and of course *all* wars were waged by capitalist or feudal powers at the time). They favored the victory of whatever nation (i.e., whatever national ruling class) would have the most advantageous consequences for the working-class movement. In particular, they wanted to see Tsarist Russia defeated, as they considered it the basis of reaction in Europe. Their anti-Russian statements were constantly cited by pro-war German Social Democrats in 1914-17.
To Lenin, of course, Marx's favoring the lesser evil (in the sense of the nation--or if you prefer, national ruling class--whose victory would do most to advance, or at least less to retard, the revolution) was an obsolete policy. This was now the age of imperialism, in which all sides in the war were equally reactionary, and the only answer was proletarian revolution. Would Marx have agreed, or would he have maintained his old anti-Russian line? I don't know, but my point is, *whichever way Marx decided* those Marxists who disagreed with him would not be persuaded. Likewise, suppose that Marx agreed on opposition to the war as an imperialist war in 1914, but disagreed with Lenin on other matters--e.g., suppose Marx agreed with not only the Mensheviks but many Bolsheviks that "defensism" was justifiable after the February Revolution. (After all, the Germans *did* come pretty close to a victory in the West which would have allowed them to liquidate the Russian Revolution.) Again, Lenin would simply say that Marx in his old age was unfortunately departing from Marxism.
My point is not to decide what the "true" Marxist position on the issues confronting the world socialist movement in the first two decades of the twentieth century would be. My point is that nobody--not even a surviving Marx!--could *authoritatively* give an answer to them. An elderly Marx's (or more likely Engels') opinion of the implications of *The Communist Manifesto* and *Capital* for the early twentieth century would simply be one man's opinion--even if that one man was the co-founder of Scientific Socialism. Nobody else would be *bound* to accept it, and those who disagreed with whatever position a nonagenarian Marx or Engels took would continue to disagree.