Themes in Semper Fi are similar to those in other Griffin novels. Loyalty is especially important, given especially the Marine Corps' tradition as an elite force.

The theme of change — rapid and dramatic change — also runs through the novel, be it a readjustment in attitudes (such as the reevaluation of Japanese fighting ability after the Pearl Harbor attack), a change in weapons (Griffin spends considerable time discussing the differences between the Springfield and Garand rifles), a change in status (both McCoy and Master Gunnery Sergeant Jack Stecker are raised from noncommissioned to commissioned rank, and Stecker will be a Brigadier General six novels in the series later).