In the current age, the concept of a planetary invasion is not entirely foreign to Earth’s space navies. While such an undertaking represents a significant challenge for any navy, the concepts behind it, and the broad details of the execution are age-old. With small-scale examples dating all the way to the age of antiquity, but mind you those are amphibious landings and not planetary landings.

Another important thing to note is that planetary invasions like their historical counterparts can come in several forms. Depending on the objectives behind the assault in the first place. There are several different types of invasion, a full assault is the most commonly considered form, but there are also raids and other types of smaller-scale operations.

Regardless of form and planning, all invasions can be boiled down into several key phases. The first phase after an invasion has been planned and launched is not landing troops on the planet but obtaining orbital control. The particulars of this can vary depending on what exactly is defending the planet. A minor farming world on the edge of an interstellar power and a scant population might not rate much in the way of orbital defenses. Such a world might not have more than a single cruiser and maybe a small station in orbit to protect it. While a core world with a massive population in the billions, and dotted with large cities and industrial centers would likely be well defended. It would likely have a fleet on station, along with orbital platforms, and perhaps even a mighty starbase. All as part of its orbital defense.

Regardless of the level of defenses in place, the first thing an invasion fleet must do. Is to sweep these defenses aside, and obtain orbital control of the planet. This would allow the next phase of the invasion to begin. Which is again not landing troops.

A defended world like a core world would likely have numerous bunkers, and ground installations equipped with ground-to-space weaponry. It may even be protected by something like a planetary shield. These defenses must be neutralized before the wide-scale deployment of troops to the surface can begin.

There are several options an invading force may choose in eliminating these defenses. The first such option is planetary bombardment. This is not dissimilar to shore bombardment as used by historical wet water navies in numerous invasions throughout history. The idea is to bring to bear naval artillery to level or soften up enemy defenses before you land your forces. The exact application of bombardment can vary again depending on objectives and goals. Be it limited restricted bombardment or widescale bombardment. Of course its also worth noting that this phase can begin while the first phase is underway if you are planning to use bombardment.

If for whatever reason bombardment is not on the table for dealing with enemy defenses. The next big option is the deployment of special forces units to the planet. The objective of such forces is to locate and sabotage defensive installations, to allow for the widescale deployment of ground forces.

The final option is certainly not preferred for most cases, and that is to simply ignore the defenses, and attempt a major landing while under fire from enemy guns. This can have a number of drawbacks but is an option if for whatever reason neither options one or two were viable. Just note that for some invasion types this isn’t an option at all, and it would be very odd to see outside of a full-scale assault.

What all of this means is that once you have orbital control, and key defenses are neutralized or ignored as the case may be. You can finally land troops on the planet. The first goal of landing is often the establishment of a beachhead or beachheads as the case may be. The purpose of these beachheads is to create a secure location from which additional forces and supplies can be landed or in certain scenarios a secure location from which forces and supplies can be removed from a hostile world.

It is also in this phase that the invasion’s form is most notable with the greatest differences between goals becoming evident. In an all-out assault for control of the planet forces once grounded would attempt to capture and hold key objectives. These may include major cities, and strategic military sites such as local starports, or ground bunkers.

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A raid of course would be much different, as the goal is not to hold territory. Holding the planet might not be the goal either. Once troops are on the ground and a beachhead is secure the goal is to acquire the objective, and get out. As such maintaining control over key objectives is not needed only acquiring whatever personnel, equipment, or supplies they were sent to capture.

Regardless of objectives, the general tools for an invasion of any scale remain the same. The most critical of which is a means to transport your troops, weapons, and supplies to the field of battle. Without these means, it doesn’t matter how great your weapons are, or how well trained your troops are, as they won’t be able to fight, and you have no business planning a grand invasion of another world.

As such let's take a look at the ships used for this goal. Transit between systems can take weeks or even months depending on your drive technology. As such interstellar vessels tend to be fairly large. Giving them plenty of room for all the supplies needed for such a journey. It also means that many capital ships have room to carry a complement of troops and equipment, and the largest often carry a fair number. However, you might want a dedicated ship for this role. In other words a troopship.

A troopship is a reasonably large interstellar vessel perhaps the size of a cruiser and dedicated to carrying troops, equipment, and supplies to the field of battle. They are not intended to engage other ships in battle, but they may be armed for their own defense, and to provide fire support to friendly ground forces. How heavily armed a troopship is may vary depending on the naval doctrines of the fleet they were designed for. Their primary role however does limit how much they can carry in terms of weapons, as they are often filled with a lot of empty space for troops, equipment, and whatever supplies they may need.

For the delivery of troops to the field of battle, a troopship can either land itself or insert troops via smaller auxiliary craft. For the initial phase of a battle, trying to land the troopship is often a dangerous and tricky proposition, and therefore secondary vehicles such as gunships, and dropships are important. Given this intent, many troopships are outfitted with sizeable hangers for such ships to which a force may attach fighter units that can be useful for a number of purposes. From defending the ship in space combat to providing air support for ground troops.

Dropships on the other hand are smaller vessels typically short-range shuttles not capable of interstellar flight. They are designed to insert troops in potentially hostile situations. They are often armored and depending on size and design can drop anywhere from a single squad to a small company onto the ground. Most dropships are between those two extremes in terms of capability. With the typical dropship being able to carry at least one armored vehicle and at least two squads. Groups of vessels like these would be used to ferry troops to wherever they are needed and are useful at all stages of an invasion.

Another important item to mention are orbital drop pods, which are expendable single-use vehicles intended to deliver a single occupant or a small number of occupants to the surface. These are most often used during the early phases of an invasion to deliver special forces akin to paratroopers as used in historical invasions like the famous landings of the second world war. These troops are meant to sow chaos, capture or destroy key objectives, and even sabotage defensive installations such as planetary shields or surface-to-space guns. Also of note, some variants of the drop pod archetype have been adapted to deliver armored vehicles to the field of battle. Some vehicles are also designed with this type of insertion in mind.

With that, we have covered but a small portion of the immensity of a planetary invasion. Yet all of the key points have been covered. Allowing us to break down an invasion into seven general phases. Some of which may or may not apply depending on your invasion type and goals.

Phase one: Planning

Phase Two: Orbital Assualt

Phase Three: Tactical Bombardment/ Special Forces landings

Phase Four: Beachhead Landing

Phase Five: Main Assualt I.E the part where you secure your objectives

Phase Six: Occupation/ withdraw depending on invasion type

Phase Seven: Conclusion