The Worldwar series at this time (Jan. 1997) consists of three books: In the Balance, Tilting the Balance, and Upsetting the Balance. A fourth is due out soon, called Striking the Balance.
It is sometimes appropriate to review series of books as a whole. This is because they sometimes read and feel like one continuous (long) book. I think Philip Jose Farmer's Riverworld series is a prime example of this. It is not always a bad thing. It depends somewhat on the content; it is forgivable if the story is good.
And such is the case here. Harry, who is the "master of alternate history" as the cover of my copy proclaims, has taken a good concept and implemented it extremely well.
It goes like this. Everything is the same up to 1942. WWII is in full swing. Nazi Germany is invading the USSR. The US is fighting Japan in the Pacific, Germany in North Africa. England is being bombed. Then, something different occurs. Aliens (like from Outer Space) invade.
Ok, the aliens being space-faring must have superior technology. And humankind is busy fighting a war. Should be no contest, right? A walkover. Not so fast. It turns out that the aliens (called the Race by themselves, or the Lizards by humans) are on a very flat technology advancement curve. The other races they know of (and have enslaved) are, too. Advances come over millennia for them, in gradual increments. They surveyed earth 1000 years earlier, and the invasion fleet was planned accordingly. What they expected was men on horseback, with spears, wearing animal skins, or at best, chain mail. They did not expect anything like tanks, aircraft, or projectile weapons. Moreover, the aliens are not expecting much resistance, and are not very good at tactical warfare (nor strategic, really). Humans, or Big Uglies as the Race calls them, on the other hand have leaders like Rommel, Patton, Zhukov, to name just a few. People who are very good at warfare. Plus humanity's steep technology curve begins to lead to horrifying rapid inventions, such as radar, and ... the atom bomb!
So it turns out not to be so one-sided after all. Rather to the chagrin of the aliens, the humans manage to unite in the face of the common enemy. Given the enmity of the times between the Germans and the Russians and the Americans and the Japanese, both of which had (has?) nasty racial overtones, Harry has to work hard to make it plausible. But he succeeds.
Like my other favorite of his, Guns of the South, Harry mixes historical characters with fictional ones to good effect. We meet historical figures such as Stalin and Molotov, Enrico Fermi, Hitler briefly. And an ensemble of fictional ones, who are very well done.
By the time this is posted, I should have finished Upsetting the Balance. Striking the Balance is not yet out. I am eagerly awaiting it. I don't know if it is the final book in the series or not. We'll see.