This is a crucial line in Nostromo's theme of the dream-ideal. The very
act of putting the ideal into practice in the physical world causes a "moral
degradation," a process we have witnessed in the lives of all the major idealistic
characters. Charles Gould, who desired justice, threatened the tyrannical destruction
of the mine; Mrs Gould, who desired reform, precipitated war; Don Jose, who desired
national rectitude, plotted revolutions and ran guns. The inevitability of this
process, the novel suggests, comes from the nature of the world, in which one
must establish one's "idea" by quashing the competing ideas of others.
The "successful action" is one that has triumphed through strength and
perseveres by oppression.