Schumpeter’s theory is interesting for several reasons. It was formulated at the same time as Lenin’s and Luxemburg’s and clearly with the knowledge of the two. It reacts to the exactly the same events as theirs. It is different though and it was held by Schumpeter throughout his life. The key text for Schumpeter’s theory is “The sociology of imperialisms” (note the  plural) published in 1918-19.