What did winston churchill mean by this?

"But if we fail, then the whole world, including the United States, including all that we have known and cared for, will sink into the abyss of a new Dark Age"

He meant that if they failed then everything that the world has worked so hard for, to advance and become "smarter" than we once were would all be for nothing. We would enter a new Dark Age. The Dark Ages were an extraordinarily difficult period in the story of humanity. It is estimated that 100 million people died at the hands of war, poverty, and plague. But during this time new ideas and ideals were born and much of the groundwork was laid for the world we know today.