The countless wars of South America, especially the series fought between Chile and Bolivia, are given almost no space. The Boer War(s) are also given little writing. I believe there are some books which focus on the last, most sporting (from the British perspective) one but that hardly counts. And during WWI, the theaters in China, the Pacific and in Africa. The Toyota War is surprisingly overlooked, even with its catchy name. Finland and the USSR and their engagements. Medieval Sweden and their expansion across and into modern-day-Germany. Wellington in Portugal. The Austro-Prussian war is always overlooked. And the Franco-Prussian war always becomes a WWI footnote. The Normans of the Byzantine Empire... The Byzantine Empire in general. Carthage. The Latin successor states to the Byzantine Empire. Trezibond has a few, but could use better.
And I could go on....