What is the main conflict in Childhood's End by Arthur C. Clarke?

The science-fiction novel, published in 1953, is a tale about the extinction of the human race. Beginning in the year 1975, the Americans are competing with Russians to be the first in space travel. The hopes of both countries are dashed, when invaders from another planet occupy Earth. The ships, operated by enigmatic creatures referred to as the Overlords, hover over every major world city.