The Glory That Was

A true tour de force for de Camp, The Glory That Was brings together many of the themes the author excelled in writing about, including time travel and alternate history.
Earth in the twenty-seventh century is ruled by a constitutional monarchy, though both the World Emperor and his Prime Minister have plans to assume greater power by neutralizing the other. In the power struggle that follows, the Emperor is ceded total control of Greece for a secret experiment that has Greece cut off from the rest of the world by a force-field and people of Greek descent being kidnapped around the whole world.
When the wife of a prominent classical scholar disappears, he vows to get her back and he and his friend plan a way inside the barrier surrounding Greece. But they are hardly prepared for what they find inside … a Greece reverted back to ancient times and under the control of a megalomaniac who will not stop before he has conquered the whole world.