Marx still believed that the west would be the first to transition to socialism.

And Marx literally directly contradicts you on this. This letter comes after the publication of Capital, and Marx is explicitly stating the opportunity to not have to become a capitalist country.

Now what application to Russia can my critic make of this historical sketch? Only this: If Russia is tending to become a capitalist nation after the example of the Western European countries, and during the last years she has been taking a lot of trouble in this direction – she will not succeed without having first transformed a good part of her peasants into proletarians; and after that, once taken to the bosom of the capitalist regime, she will experience its pitiless laws like other profane peoples. That is all. But that is not enough for my critic. He feels himself obliged to metamorphose my historical sketch of the genesis of capitalism in Western Europe into an historico-philosophic theory of the marche generale [general path] imposed by fate upon every people, whatever the historic circumstances in which it finds itself, in order that it may ultimately arrive at the form of economy which will ensure, together with the greatest expansion of the productive powers of social labour, the most complete development of man. But I beg his pardon. (He is both honouring and shaming me too much.) Let us take an example.

In several parts of Capital I allude to the fate which overtook the plebeians of ancient Rome. They were originally free peasants, each cultivating his own piece of land on his own account. In the course of Roman history they were expropriated. The same movement which divorced them from their means of production and subsistence involved the formation not only of big landed property but also of big money capital. And so one fine morning there were to be found on the one hand free men, stripped of everything except their labour power, and on the other, in order to exploit this labour, those who held all the acquired wealth in possession. What happened? The Roman proletarians became, not wage labourers but a mob of do-nothings more abject than the former “poor whites” in the southern country of the United States, and alongside of them there developed a mode of production which was not capitalist but dependent upon slavery. Thus events strikingly analogous but taking place in different historic surroundings led to totally different results. By studying each of these forms of evolution separately and then comparing them one can easily find the clue to this phenomenon, but one will never arrive there by the universal passport of a general historico-philosophical theory, the supreme virtue of which consists in being super-historical.