Childhood's End by Arthur C. Clarke.

Childhood’s End is set in the period immediately following World War II. In postwar Russia and America, German scientists head separate programs to begin to explore outer space. Both programs are within weeks of launching their first rockets when enormous alien ships appear in the skies over Earth’s major cities. For the first third of the novel, “Earth and the Overlords,” the aliens remain in their ships, and their leader, Karellen, the Overlord who is designated Supervisor of the Earth, will deal directly only with Rikki Stormgren, the secretary-general of the United Nations. The aliens maintain that they are too frightening to show themselves and that they will remain in their ships for fifty years—a period they feel will allow humans to grow accustomed to them and their benign intent. The Overlords interfere with Earth’s affairs in only two ways. First, they refuse to allow any kind of space program that might lead to human exploration of other planets. Second, they abolish all international rivalries, leaving governments to handle only local (internal) affairs. The Overlords do take a few specific actions. They force South Africa to end its divisive racial policies, and they also force the Spanish to abolish bullfighting.