The general understanding of the United States in Marx's historiographic vision is fairly straightforward, however. The American Revolution was, like its counterpart in France, a bourgeois revolution, which had the social aim of overthrowing the feudal-aristocratic order and imposing the rule of the capitalist middle classes, or in Marxist terminology the bourgeoisie. What made the American Revolution special for Marx, however, was that feudalism had no real roots in America in the first place, it was "subordinate to bourgeois society" from the very beginning; this, combined with the fact that America was a whole new continent available for exploitation, allowed the bourgeoisie to "develop to hitherto unheard-of dimensions". [1]

As he makes clear in On the Jewish Question, this is also reflected in the United States' commodification of religion. In the US, "the relation of religion to the state" is crystallised in its pure form. "The preaching of the Gospel itself and the Christian ministry have become articles of trade, and the bankrupt trader deals in the Gospel just as the Gospel preacher who has become rich goes in for business deals". [2] The bourgeois revolution in the United States has subjugated and commodified religion to an extent not seen anywhere else.

In 1846, in fact, Marx described America as the "most progressive nation" on the planet. [3] Now, this is a rather ambiguous quote because he is actually describing what he thinks Proudhon says about America, but if you read the passage as a whole it seems clear he agrees on that point, just not the idea he ascribes to Proudhon, namely that racial slavery is well and good because it sustains American economic life.

Marx supported the intensification of bourgeois relations, as in the United States, as a prelude to proletarian revolution: "In a word, the free trade system hastens the social revolution. It is in this revolutionary sense alone, gentlemen, that I vote in favor of free trade." [4]

Interestingly, later on, Marx actually put his name to a letter to Abraham Lincoln, which was submitted by the International Working Men's Association in 1864 and received by the American government. The text of this letter is reproduced here, and is worth a read since it's very short. Of particular interest is the final paragraph:

The workingmen of Europe feel sure that, as the American War of Independence initiated a new era of ascendancy for the middle class, so the American Antislavery War [i.e., the Civil War] will do for the working classes. They consider it an earnest of the epoch to come that it fell to the lot of Abraham Lincoln, the single-minded son of the working class, to lead his country through the matchless struggle for the rescue of an enchained race and the reconstruction of a social world.

Also of interest is that Ambassador Adams' reply on behalf of Lincoln is cordial and seems to accept the sentiments of the letter in full!

Sources:

[1] In Bruce Cumings, 'Revising Post-Revisionism, Or, The Poverty of Theory in Diplomatic History', in America in the World: The Historiography of US Foreign Relations Since 1941, p. 48.

[2] http://www.marxists.org/archive/marx/works/1844/jewish-question/

[3] http://www.marxists.org/archive/marx/works/1846/letters/46_12_28.htm

[4] http://marxists.org/archive/marx/works/1848/01/09ft.htm#marx