I am wondering what the connection between these three topics were because Historian Carl Degler said that they were part of a revolution. Particularly, imperialism seems out of place when put beside the other two domestic stages of change in American history. Can these events be put in the context of a revolution in that it has a moderate stage, followed by a radical stage, followed by a conservative stage, like the French Revolution?
They're better seen not as a succession of stages - each overlapped with others - but as manifestations of - or reactions to - the Gilded Age transformation of the US from an agrarian-based society into one dominated by urban industrial and commercial interests. Populism was very much a response on the part of those who saw the country at risk from rising business and political elites, Progressivism an attempt to realise the potential of the new era for social improvement and better government. Imperialism is indeed the outlier, an expression of the very interests Populists had railed against, but Populism and Progressivism too differed too much in their social base, principles and methods to be seen as a succession. The underlying revolution was not so much political as economic and social, all three movements reflecting its maturation as the US far surpassed all rivals.