Student or Learner
Many varieties went about their daily affairs clad in the armor of a medieval knight. Others were invisible to the human eye but they multiplied at such a terrific rate that they would have owned the entire earth in less than a year's time if it had not been for certain enemies who were able to destroy them almost as fast as they were born.
From Van Loon's Geography
Does "for" as a preposition mean "show a reason or cause" here?
Can I rephrase it like:
They would have owned the world if certain enemies have not destroyed them almost as fast as they were born.
Yes, you have the right sense. You can replace "for" with "because there were" in this sentence. Your last sentence is almost right, except for "if certain enemies had not destroyed...."