One of my favorite answers to this question involves my love of military sci-fi of the 19th century.

You see, between the Franco-Prussian War and the outbreak of World War I, Great Britain had a flourishing genre of what was known as invasion literature. Stories within this genre typically involve some foreign enemy attacking and invading the British Isles, bringing low the mightiest empire (as the British thought of themselves) on the face of the planet.

The Battle of Dorking: Reminiscences of a Volunteer is a novella published in 1871 by George Tomkyns Chesney and is considered to be the prototypical bit of invasion literature. Chesney was in the Royal Engineers and thought that Britain was woefully under-armed to defend itself in the event of a full-scale war. His views, particularly after the speedy German victory in the Franco-Prussian War, were popular among conservative circles. Having won a grand empire, conservatives most feared a change in circumstances that might cause that empire to disappear.

In any event, Dorking is told from the point of view of a veteran, 50 years after the war that ended the British Empire. It describes how the enemy (unnamed but presumed to be German) used secret wonder weapons and landed an invasion force in Essex. Britain mobilizes a force of volunteers to resist the invaders, but poorly trained and badly led, they are routed by the enemy.

Britain becomes a province of a foreign empire, with Germany taking Malta and Gibraltar, Canada and the West Indies going to the United States, and Australia, India and Ireland all becoming independent.

The most famous example of invasion literature is, of course, The War of the Worlds, but that's purely science fiction and not military fiction. My favorite piece has to be "Danger! Being the Log of Captain John Sirius," a short story published by Sir Arthur Conan Doyle and published in July 1914. Doyle wrote it in 1912, and the story describes how a small European country successfully wages a war against Great Britain using a handful of submarines. It's well-written, and with the benefit of hindsight, incredibly captivating since we know what will happen a handful of years after Doyle writes it.

While Dorking was the first really big bit of invasion literature, military SF in English goes pretty far back. In 1763, there was a book called Reign of George VI, 1900-1925. It was by an anonymous author, but is a pretty creative bit of military fiction that covers the titular warrior king's campaigns as he becomes ruler of Europe, king of France and commander of a massive overseas empire.

Unfortunately, it doesn't forecast any big changes in technology. All the fighting features ships-of-the line, musketeers and cavalry. There is no levee en masse, officers are all aristocrats, and kings or emperors lead from the front lines.

If you're interested in military SF of the past, I recommend picking up The Tale of the Next Great War, 1871-1914, edited by Ignatius Frederick Clarke, which is a fun survey of invasion literature between 1871 and 1914. It's a good place to get started and it's sure to get you hooked if you have even a smidgen of interest.