The theme of "If I Forget Thee, O Earth" by Arthur C. Clarke is that if humans continue to pollute and bomb the Earth, they will drive themselves into a long exile from their home planet. The story was published during the 1950s, when nuclear warfare was a hot-button issue.
A secondary theme of the story is that future generations must be taught to respect and protect the Earth. Marvin feels the weight of responsibility as his father reveals to him the true damage humans have inflicted upon the Earth. Marvin sees the interaction as a rite of passage for boys in the Moon colony and envisions a far-off future in which the human race can return to their home.