The philosophers have only interpreted the world in various ways; the point, however, is to change it. [Theses on Feuerbach, 1845]
The history of all hitherto existing society is the history of class struggles. [Communist Manifesto, 1848]
History repeats itself, first as tragedy, second as farce. [The Eighteenth Brumaire of Louis Bonaparte, 1852]
In every revolution there intrude, at the side of its true agents, men of a different stamp; some of them survivors of and devotees to past revolutions, without insight into the present movement, but preserving popular influence by their known honesty and courage, or by the sheer force of tradition; others mere brawlers, who, by dint of repeating year after year the same set of stereotyped declamations against the government of the day, have sneaked into the reputation of revolutionists of the first water.... They are an unavoidable evil: with time they are shaken off. [Address of the General Council of the International Working Men?s Association on 'The Civil War in France,' 1871]
Funny how that last quote sounds like it could have been written far more recently than 1871, isn't it? Not like we're thinking about any people in particular. Certainly not people in the US.
There's a good short bio of Marx here.
You'll find a longer biographical sketch (including a bibliography) is here at Wikipedia.