Most science fiction aliens get more than they bargain for when they try to invade Earth.

Harry Turtledove's new "Colonization" series is no exception, but he takes the conflict between human and alien a step further by exploring the consequences of a decades-long balance of power.

Colonization: Second Contact begins a sequel to Turtledove's "Worldwar" tetralogy, an alternate-history epic in which an invasion of reptilian aliens interrupted World War II.

The aliens, who call themselves "the Race," expected the invasion to be easy -- their scouts had surveyed Earth 1,600 years earlier and found humans to be hopelessly primitive.

The Race still had a technological advantage, but over four books the humans turned the invasion of the planet the aliens called "Tosev 3" into a nightmare. After a limited nuclear exchange, both sides realized they were more likely to destroy the planet than win the war, and an uneasy truce was struck.

Second Contact picks up the story in the early 1960s, two decades years after the invasion began. The United States, the USSR, Britain, Japan and the Greater German Reich remain under human control, while the Race controls the less-developed regions of the world.

As the book opens, a second wave of alien colonists arrives at Tosev 3. While they have no military equipment or training, their sheer numbers threaten to shift the balance of power in favor of the Race.

The second wave also poses problems for the invasion force. The colonists expected to wake up from cold sleep to find a pacified planet waiting for them, and they have none of the invasion force's hard-won experience with the adaptability and unpredictability of humans.

Meanwhile, the free governments of Earth are doing their best to stay that way. Even working together, they can't hope to throw the Race off the planet, but each nation has a strategy to survive.

Heinrich Himmler's Nazi Germany relies on science and the threat of a second, nuclear holocaust. The Russians quietly support Communist insurrectionist Mao Tse-Tung's rebellion against the Race in China, and the Americans are developing an top-secret space station project.

The humans also try to exploit the Race's unexpected weakness for ginger. The innocuous root's druglike effect on alien physiology nearly derailed their military machine in the 1940s, and now threatens to wreak havoc with the colonists' social order.

Turtledove's focus on the "ginger problem" highlights a parallel between the series and real history. In many ways, Tosev 3 is the Race's Vietnam.

To the Race's common soldiers, Earth is a miserably cold battleground for a war that they have no personal stake in. In the "Worldwar" series, ginger became a refuge from the war for the troops; once the colonization fleet arrives, the drug becomes a destabilizing force in the Race's civilian society.

Vietnam is only one of many echoes in the story, though. Turtledove looks at the lives of humans and aliens across the planet, showing how the Race's arrival has changed all the world's societies.

What's surprising is how accepted partial alien control of Earth has become in 20 short years. The older characters in Second Contact -- many of whom were in the previous "Worldwar" series -- may be dedicated to defeating the Race, but their sons and daughters are adopting the extraterrestrials' culture as "chic."

The more advanced Race proves less eager to adopt human customs. Instead, Turtledove shows how their increasing awareness of the human perspective leads them to question the flaws in their own society.

Turtledove isn't a particularly good writer -- his dialogue is flat and his characters frequently spout large chunks of exposition. He's an excellent historian, though, and his extrapolations of 20th century history with aliens are plausible and entertaining.

While the kind of finely balanced invasion scenario Turtledove suggests seems unlikely, "Colonization" raises fascinating questions. Historically, China has been so vast that it ultimately absorbed wave after wave of conquerors -- might the same thing happen to invaders on a planetary scale?