Complex sentence: Percy Greg, like his father, wrote about politics, but his views were violently reactionary: his "History of the United States to the Reconstruction of the Union" (1887) can be said to be more of a polemic, rather than a history.
Split into simpler sentences: Percy Greg, like his father, wrote about politics, but his views were violently reactionary. His "History of the United States to the Reconstruction of the Union" (1887) is a polemic rather than a history.