His novels range from epic fantasy (Oath of Swords, The War God's Own) to space opera (Path of the Fury, The Armageddon Inheritance) to alternate history (1632 series with Eric Flint) to military science fiction with in-depth characterisation.
Many of his stories have military particularly naval themes. By frequently placing female leading characters in what have previously been seen as traditionally male roles, he has explored the challenges faced by women in the military and politics.