In the mid-19th century, Central Europe was in flux. The map had been torn apart by Napoleon I, and glued back together by the Congress of Vienna, and in every state the effects of the Industrial Revolution was being felt. Onto this stage strode one of the true giants of world history, Otto von Bismarck. Son of a minor Junker landowner, and possessed of an iron will, von Bismarck climbed the rungs of the Prussian state until he was the effective head of the nation. But he wasn't satisfied. By winning two wars he had no right to win against the vastly superior Austrian and French Empires, he forged a new united German state.
And then he stopped making wars and turned to diplomacy. Because von Bismarck understood what I stated above, that wars have to have a reason and a goal. The Imperial Chancellor hated wars, as they were something that was out of his control. But he still kept them as a possibility, as an instrument of state policy if needed. But even for this master statesman who created and ran Germany for 40 years, there was one line he would not cross unless forced: Russia.
Speaking to the German Ambassador to Vienna in 1888, Bismarck stated that one would not actually defeat the Russians. “The most brilliant victories would not avail; the indestructible empire of the Russian nation, strong because of its climate, its desert, its frugality, strong also because of the advantage of having only one frontier to defend, would, after its defeat, remain our sworn enemy, desirous of revenge, just as today’s France is in the West."
Which is relevant to my novel, as I'm writing about a massive, multi-sided war out among the stars. But we don't live in deep space, we live on planets. And what von Bismarck had to say about Russia applies ten-fold to taking an entire world. Unless there is only a small colony present, or the inhabited body is incapable of supporting life and can be threatened with destruction of life support facilities, an invading force has only two real options.
The first is the doctrine of total war. Since you can't control the planet, render it useless to the enemy. Destroy orbital facilities, cripple the surface by hurling projectiles at a small fraction of the speed of light at vital points (as energetic as a nuclear device, with no radiation. So your colonists can move in when the dust settles.)
This doctrine is needlessly destructive, and is usually only resorted to by desperate commanders or in reprisal for similar attacks by the enemy. No, if you want to hold a world, exploit it, rule it, you have to put boots on the ground.
But we just established that conquering a planet is impossible! True, but you can control enough of it to maintain effective control of the rest, just like a police officer can neutralize a comabative subject with the right twist of a wrist. This is called nodal warfare. The idea is simple. Control or disrupt the important points that allow a civilization to function (the nodes) and you gain control by proxy over the rest of the planet.
Some nodes are obvious. Government centers, major cities, transportation hubs. Others may be less obvious. Gain control over the communications network, and you not only can make sure the population hears what you want them to hear, but you also make it hard for resistance cells to operate. Control the sources of food and water and rebels face a desperate choice. There are others, but I think the idea is clear.
This invasion and occupation would be supported by one or two orbiting cruisers, or perhaps a small squadron of destroyers. They're they to both coordinate efforts on the ground, and provide retributive fire if needed.
The final key is popular opinion. If the previous government was oppressive, the invaders may be greeted as liberators . . . or not. (See George W. Bush and the invasion of Iraq, 2003.) If the occupation is handled well, and a strong propaganda campaign is in place, you might turn popular opinion your way.
But still. Don't invade Russia. It's a bad idea.