Space Warfare

At present it is not possible to engage in battle in hyperspace as essentially each ship is in its own little universe. hyperspace impacts on space warfare in two major ways, firstly the fact you need to achieve 0.25C to jump means trips take at least a week and when you de-transistion you have a velocity of about 0.1C .  
Normal Space
Geography becomes more complex in space. Space is open, and vast. The openness of space makes it harder to exploit the "terrain". If you can be seen, you can be hit. There are very little that can serve as cover. Planets, asteroids, large habitats and orbital junk gain strategic value by their ability to obscure parts of space in addition to their potential as bases or objectives.
The size of space is the factor that truly changes space combat from traditional combat. It is both a question of the distances involved, time dealys and its three/four-dimensionality.
Distances being large, ships must use high speeds to reach targets, which means that when they meet they have significant relative velocities making contacts brief. An object nearly stationary in the reference frame of one ship will impact the other with a high speed, possibly doing fatal damage. However, since the distances are so vast physical contact is nearly impossible to achieve by accident
In three dimensions many more possible places become available to a moving ship after a certain amount of time than if it had been travelling two-dimensionally on a surface or even worse, along a one-dimensional road. This greater manoeuvrability translates into the "slipperiness" of space combat. It is impossible to hold lines, block or pin down the enemy, while evasive actions, circumnavigation and quick escapes are easy.
The size and dimensionality of space makes it possible to hide using distance. If an enemy doesn't know where you are and you do not emit any telltale radiation, the search volumes can grow so large that contact is nearly impossible. If a ship has sensors that can detect another ship at a million kilometres distance, and the hidden ship is within a 1 cube AU volume, the chance of finding it by sampling a random position is little more than one in a million. In interstellar space distances are so large that it is practically impossible to find anything hiding, something which has been exploited since the dawn of interstellar travel.
The size of interstellar space makes it possible to send entire fleets through enemy lines with little chance of being discovered before reaching their target. Borders have no meaning, blockades are nearly impossible unless tightly surrounding a planet or habitat.
To make matters even harder, space is truly four-dimensional. Everything is moving, and it is not just distances that matter but also their relative speeds and timing.
In a two-dimensional battle it is possible to encircle an enemy, but in space it is usually nearly impossible to englobe an enemy. The number of ships needed would be large, and as a general rule it is always possible to turn around any block in space. What can be done is to exploit orbital dynamics, so that placing a long-range weapon in the preferred path towards a target will force enemies to either move through the range of fire or bend away, forcing them into slow or less useful paths. This strategy was very successful in the interplanetary era as ships could not waste much fuel and largely moved along low-acceleration trajectories. Today many ships can spend enough fuel and acceleration to get back into the right trajectory, but the method still tends to slow them and can buy defenders valuable time.
Space combat is actually combat in phase space, the six dimensional space of position and velocity. The leader that can exploit the peculiarities of the local phase space will have a great advantage. Gravity acts by bending trajectories, different velocity vectors enable different kinds of attack and the asymmetries induced by stars and planets in visibility provide great creative material.
Planets, megastructures, orbital habitats and asteroids are sitting ducks. Their positions can be known with great accuracy, and they cannot dodge any incoming fire. Although planets and some megastructures are protected by their sheer bulk, in general such major structures are too vulnerable to have much military value. Instead military installations are kept small, mobile and placed in awkward orbits or deep space in order to make them hard to reach. Major structures in general are kept almost totally civilian in order to avoid drawing enemy fire, although in some cases more ruthless polities include sensitive military systems in the highest population regions hoping enemies are too humane to attack them. Most armaments of planets and habitats are intended for policing as well as deflecting incoming meteors and junk.
Information and Logistics
In space combat information is the scarcest commodity. The nature of space combat makes communications hard. It is not just the demands of stealth that makes it hard to communicate, but also the long delays. If the battle moves fast, messages will not be able to catch up with a fast ship before it is too late.
Lightspeed delays means ships have to wait for long periods of time before getting information about the status of each other. This makes giving orders even more complex, as an order may be received when it is no longer relevant or new events have superseded it. One solution that is commonly used is order sets, detailed instructions combined with information about what conditions the different orders are relevant. They act as programs for what to do when and where if something happens. But their performance is only as good as the strategy of their creator, its grasp of likely events and how they are implemented by the individual ships. In the end, space combat always depend on the abilities of individual ships rather than any central general - just as in economics, the centralised leader will lack enough information to make informed decisions, while the individual agents might lack overview but at least know what is going on.
Autonomous weaponry is a logical outcome: devices and ships acting on their own. But there is no fixed border between a tool, an autonomous device and a soldier. What happens is that leadership by necessity has to be distributed. In Sun Tzu's day the voice volume of the general was an important factor: if he could not be heard, he could not be obeyed. Today every ship has to be its own general - or contain a copy of the general.
Detecting where anybody is without being found yourself constitutes the central problem. It is the key to successful attack as well as defence. A ship in stealth mode is hard to discover without using an active sensor like a radar or ladar, and that will reveal the searcher's location like shining a beacon. Some arms systems use disposable radar drones to determine where everybody is, and then tightbeam it to the ships. But in general active sensors are vulnerable to countermeasures, jamming, radiation from explosions, hiding behind objects or in the vastness of space.
Passive sensors are harder to use but often necessary. The existence of a ship is impossible to hide perfectly, but discovering them in the world of modern stealth and deception technology is hard. Reflected electromagnetic radiation, heat emissions, plasma trails, neutrinos from drives, gravity waves from fields and occultations of remote stars can all reveal a ship's location, but can also be hidden using various means. These means can be pierced, but you have to know what to look for - creating yet another spiral of deception and counter-deception. Any piece of information about enemy capabilities and strategies is valuable as it helps figuring out what systems to look for, but look out: it could also be deception.
The best defence against this is not to give the enemy time to discover where you are. If you move at a relativistic speed, you move nearly as fast as your information. The enemy has very little time to prepare for you when you suddenly appear. The same goes for relativistic missiles, the bane of slowly moving targets. A missile moving at 0.99c will give the target only 1% of the flight time to prepare itself if the launch can be seen, and far less if it has to be detected by a sensor which then beams back the message. Combined with low-profile antimatter weaponry the result is nearly invisible weapons of mass destruction that in practice cannot be avoided if you don't move relativistically. Secure communications can be created with quantum cryptography, but the bandwidth is extremely limited. Like tightbeam messaging it also relies on both parts knowing where they are. If the ships do not know where their friends are, they cannot contact them without revealing their location. This is another factor contributing to the democratisation of space combat compared to planetary warfare.
The most feared aspect of information is perversion: being infiltrated with enemy code. Especially an enemy of a higher singularity level is often capable of insinuating dangerous information in broadcasts. It may not be subversive software per se, but could just as well be memes or disinformation. Many ships have extremely rigid security around their communications systems, and may even refuse communication except according to some pre-set schedule.
In a battle for a solar system, orbital superiority and information superiority have to be achieved. Without your control over space, the enemy will be able to go anywhere while you can't. Without your control over information, the enemy will know and you won't.
The basic question in space combat is specific impulse: how much acceleration can a ship achieve how fast for how long? This determines how quickly the ship can make its future position in phase space uncertain, as well as where it can reach. In the old days of the interplanetary era impulse was low, and space combat took very long time and involved predictable trajectories. Later isp increased to the extent of enabling fast classical space combat, and finally to the level that space combat is strongly relativistic. Each of these forms of combat work differently. In relativistic combat information about the current state is hard to discover, in fast classical combat stealth, shielding and weapons are paramount and in the old slow combat planning and surprise.
The fusion drive changed many of the rules. Ships now accelerated towards each other using high isp, launched drones and missiles able to accelerate even more strongly and in short battlebursts the military software and hardware settled the struggle.
Ships could not dodge and were easy targets, so shielding expanded. The problem was that shields cannot protect against everything, so the nature of the enemy's shielding often determined which weapons were effective. Some groups have vastly different approaches, mainly due to superior technology.
Weapon Systems
Nuclear weapons are less efficient in space than on planets, because ships are commonly well shielded and the radiation is quickly dissipated in the vastness of space. The X-rays that would have created a fireball on a planet will now just radiate away. Unless the detonation is extremely close to the ship (within tens of kilometres) it will merely be temporarily blinding and set up an obscuring cloud of debris (which can be useful). However, bomb pumped X-ray laser weapons are much more deadly
Drone weapons and missiles with limited autonomy are the most common weapons in modern space combat, since they can get close to the target and make local decisions fast enough to hit it. They can also set up a complex information network of allied equipment for one's side to exploit, although this may also be vulnerable to infowar.
Space mines have been developed, and can be used to protect strategic points in phase space. They are little more than stealthy targeting and weapon systems that wait for something to pass within range that does not have the right IFF code, and then reveal/ themselves in an attempt to destroy the target. Conventional beam, missile and nuclear mines are fairly accepted.
Beam weapons are mainly useful for defending against approaching missiles rather than killing enemies, unless they can be launched at close range by a drone.
Modern fleets able to replicate most resources during a campaign. Given enough energy from solar panels and matter from an asteroid nearly anything can be built. This makes space wars less dependent on strong industrial capacity at home, as the fleets can "live of the land".
Replicator weaponry is the logical outcome of flexible autonomous industrialisation. When it is used, time becomes essential: the group that first seeds a system with its technology will