It's not the only conflict, of course. The series is global -- interplanetary -- in scope, and Turtledove's plot ranges from Germany and Poland to Palestine, South Africa, China, California and even the asteroid belt.

His main interest, however, is exploring what happens when immovable ideologies meet an irresistible force. Down to Earth is driven by fanatics of several kinds, and their struggles with the Race raise the stakes of the entire series.

The Race has the greatest advantage in the arena of politics, as represented by their battles with Mao's communists in China.

While the Red Chinese enjoy initial successes against the slow-to-react Lizards -- even throwing the "scaly devils" out of Peking -- they are no match for the Race's heavy weaponry and undemanding regime.

Once the Communists retreat, their grassroots support in the villages rapidly erodes.

Turtledove's narrative suggests that while the Communists may have triumphed against the internal opposition they faced in our history, they wouldn't have had the resources to defeat a powerful outside threat.

The Communists' ideology even proves to be a liability. Convinced that the historical dialectic predicts victory, one of Turtledove's protagonists takes a foolhardy action that results in the capture and internment of all the Red Chinese characters.

Human religion proves much more dangerous to the Lizards, especially when religious leaders inspire violent Islamic fundamentalism. As a result, the Race soon finds itself under constant attack from gunmen and truck bombers in the Middle East.

The Muslims can't do much real damage, but they drive the Race to a major strategic error. In an attempt to discourage human "superstitions" and encourage their own tradition of emperor-worship, the Lizards impose a tax on anyone wishing to enter a human church.

In this case, the irresistible force of alien pragmatism meets the immovable object of human faith. The tax backfires badly, stiffening resistance among the most troublesome religious sects.

It also spoils relationships with some of the Lizards' best allies. While the Jews of Poland and Palestine remember that the Lizards stood between them and obliteration at the hands of the Nazis, the new taxation is a vivid reminder of past persecutions.

Meanwhile, the Nazi threat continues to overshadow all these developments. The Greater German Reich grows steadily more belligerent in Down to Earth, ultimately launching a conventional and nuclear assault on the Race-held buffer state of Poland.

Although the flashpoints of the conflict are clear, Turtledove never quite explains what drives the Reich into a new war with the Lizards. Of course, he may not have to -- Hitler's assault on Russia didn't make much sense either, and his successors in Down to Earth are no more rational than he was.

The resulting war is short and vicious.

The Race is badly hurt, but the Reich is annihilated. Western Europe is ruined, and most of Turtledove's European characters are left in jeopardy.

There's no qualitative difference between this and Turtledove's other scenarios of fanaticism. This one differs only in degree: the conflict is more virulent, and while Germany loses, there's no winner.

That's a disturbing prospect. The destruction of Germany does nothing to reduce tensions between humanity and the Lizards -- the two species are colliding on too many other fronts.

Moreover, the war underscores the determination of the Lizards. They told the Reich what would happen if it attacked, and when the Nazis struck, the Race did exactly what it promised to do. They struck back.

Afterward, the Lizards announce that if humanity seems likely to threaten the rest of the Race's Empire, the aliens will destroy the entire planet -- including their own colonists -- to stop them. The fate of Nazi Germany proves they're not bluffing.

There are those among the Race and the humans who believe there's a way both species can live together. As we hit the halfway point of the "Colonization" series, the question is: will they find that way in time?