The following reviews are as spoiler free as I can make them, while still being able to make a few comments, of the first three books of the Frontline series by Marko Kloos.
The three novels of the Frontline series Terms of Enlistment, Lines of Departure and Angles of Attack) follow Andrew Grayson and his service and adventures in the military of the NAC (North American Confederation) in the early 22nd Century. The future Earth depicted here is a near dystopia, massively overcrowded, with the two primary power blocs, the NAC and the SRA (Sino-Russian Alliance) in a constant state of cold-to-warm war (other nations and groups, for example the EU and SAU -South American Union-, do not show up until the third book and then only in essentially a cameo). It is implied from textual evidence that they was at least one more world war with limited use of weapons of mass destruction, leading to the banning of using such on Earth or the colonies. Yes, colonies, an inexpensive form of FTL travel has been developed and humanity (in the form of NAC and SRA settlements) have spread to the stars.