In the alternate history genre, it's commonplace to have historical figures as important characters. It's far less common, however, for the author's characterization of those historical figures to be based upon their firsthand knowledge of them. As a physicist who knew personally some of the leading figures of the Manhattan Project, Gregory Benford is one of the select few for whom such an accomplishment is possible, and he employs it to full effect in this novel exploring the war that might have been.
Benford takes as his point of departure the use of centrifuges to separate the U-235 from uranium hexaflouride. As he explains in the afterword to the novel, this is the primary means most nuclear powers today obtain the critical isotope for building atomic weapons, yet in 1942 it was abandoned for what proved the far less effective method of gaseous diffusion. Edward Teller was among those who theorized that had the centrifuge process been used, the United States would have obtained sufficient material to build an atomic bomb in 1944 rather than in the following year.
Benford's scientific knowledge gives him the foundation for establishing an extremely plausible premise, yet it is skills as an author which turn this premise into an entertaining work of fiction. Building his novel around the pivotal figure of Karl Cohen (who was Benford's father-in-law), he walks readers through the development of a more efficient atomic bomb program, one that has a bomb ready to use in concert with the Normandy invasion. In a lesser author's hand the reader might get bogged down in the details of the physics and chemistry of nuclear weapons development, yet Benford knows how to interweave intelligible explanations of the science with plot and character development in such a way as to keep the reader engaged. His postulation of the historical effects of the use of such a bomb are a further tribute to his ability, as instead of a Pollyanish outcome he works through some of the likely ramifications of using a weapon upon an advanced industrial power capable of responding in kind. It all makes for an alternate history novel of the first rank, one that deserves to be regarded as one of the finest examples of its kind.