Oof, making me think some of these recommendations may be a big lightweight!
1776 by David McCullough is an old favourite. As the title suggests, deals with the early part of the American Revolutionary war. Devoured that poolside years ago and have reread a couple of times.
The First Crusade by Thomas Asbridge, but I've been fascinated by the Crusades since studying them in Uni, so that may colour my opinion. Same goes for Crusaders by Dan Jones.
Rubicon by Tom Holland, about the Roman empire.
I usually enjoy anything by Max Hastings, although I've mostly stuck to his books about the Second World War.
The Walls Have Ears by Helen Fry, about the intelligence operation to glean information from Nazi prisoners of war. She's got another couple more about military intelligence I'm meaning to pick up.
Non-military book, but I was recommended Team of Rivals by Doris Kearns Goodwin after really enjoying the film Lincoln. It's a weighty tome, but I'm enjoying it at the moment.