"Have you ever seen a textbook USDF space-to-ground assault? A beautiful sight. Ships, glimmering in the rays of an alien sun, raining the fierce, righteous fury of Mankind unto the planet, hammering its soil with tungsten rods as the atmosphere quakes from the sheer impacts. Then, like a wave of steel, the pods follow of the first landing. The few, the proud, descending like angels, singing the song of war. Within hours, the first shuttles land, bringing in perfectly calculated, stored and shipped war material and soldiers. Each one of them an apex of his kind. Have you ever seen it? Well, I served this bloody outfit since fourty years and I ain't have it seen either. There is no such thing as textbook."
SolGov is a rather large interstellar power whose assets are jealously guarded by its constituents. To protect the enduring prosperity of all humans under its flag and to ensure its will is projected into the stars beyond, the United Solar Defense Force is the bulwark of SolGov and their federal armed force.
The expansion of humanity under the efforts of the UEG first saw limited need for armed ships and outright violence, with militias often raised at ad hoc, with mercenaries, local police ships and press ganged pirates serving as the primary defense of the fledgling interstellar human community. But with the rapid growth of some colonies and humanity's natural strife under each other, open conflict began to rise- Splinter groups, separatist movements and ideological extremists found fertile grounds in fringe colonies for an armed uprising or worse. At the behest of the public the then Council of Nations stepped forth and instituted the Solar Militia Mandate, the birth of the so called "Colonial Marines".
The mandate demanded that each colony or colony cluster must have their own armed forces, trained and equipped properly for the needs of their respective territory, from which the UEG had the right to draft regiments and fleets ad hoc, based on critical needs within the controlled space of humanity. First lauded as significant improvement to the security of humans within the sphere of influence of the UEG, the failings of the system became clearer and clearer as humans expanded across space.
The requirement to have permanently equipped and trained personnel at hand resulted in a sharp rise of Private Military Contractors which gained Legal Combatant status as result- Many colonies did not want to permanently enlist soldiers in the amounts they needed due to cost. So only a nominal core of soldiers were actually permanently employed, with the rest being mercenaries, hired for the current needs of the colony. While very profitable, this led to the exact same problems mercenaries brought with them back in the ancient history of Earth.
Furthermore, many colonies did not exactly adhere to the rules mandated - Many rich colonies hired more soldiers than needed to simply force their own population or other colonies to submit to their whim. Others did not do anything at all, feeling very secure within their positions behind more entrenched and armed up colonies. Fringe colonies, however, were often too poor to adequately equip and maintain armed forces. Their colonial marines were little more than guerillas with antique weaponry.
With the rise of the United Systems of Sol Central, the Solar Militia Mandate was revised and finally tossed. The armed forces of humanity needed revision- the United Solar Defense Force were born. With the Act of Colonial Security, every colony is now required to pay the budget of the Armed Forces, dependent on their GDP, while simultaneously allowing every citizen to join these forces if desired. Furthermore, SolGov reserved the right to establish military bases and posts wherever they desire, as long the Colonial Assembly ratifies their decision.
While many Colonial Marine organizations disbanded or absorbed into the new USDF, the Solar Militia Mandate is still in effect, although much diminished - It is no longer required for a colony to have their own military and many are not able to pay for the USDF and their own force. Some do. Few actually exercise this right.
The USDF is assembled out of four branches, each with their unique doctrine,command structure and logistic lines - much like any conventional army.
Rapid Deployment Guard
"We are the shield of Sol."
The RDG is the sector-level naval guard of SolGov. Originally a part of the SDN, it gradually became its own branch due to very different logistical needs and differing doctrine - The RDG is primarily concerned with patrolling the shipping lines of humanity, as well border patrol and any stellar occurrence that needs a quick response. With fast, small ships they are able to rapidly deploy from their bases and respond as first line of defense against any incurring threat, where they delay and harass the enemy until the SDN can arrive. Their organisation is heavily decentralized to enable autonomous operation within their given sectors. In times of war, the RDG becomes adjunct to the SDN and serve as their escort ships and skirmish fleets.
"On the wings of Vengeance."
The SDN is the Navy of the USDF. Little changed in their doctrine and they are still considered the core of human military might, as any given ship of the SDN is a veritable powerhouse of defensive measures and raw firepower. An united spearhead attack of a SDN fleet can level the board in any military engagement. The problem arises bringing them there, as their ships are universally heavily armored, slow and costly to run without easy logistic lines. Deploying the SDN in a conflict usually means serious business, as their use is otherwise prohibited. SDN vessels are also used as flag ships of the RDG sometimes, in case a solid backbone is needed within a given sector.
Orbital Drop Troopers
A department of the SDN, although sometimes seen as "Fifth Branch" by particularly prideful members is the ODT. Originally used for space-to-ground assaults, the OTD has grown to become the multi-tool of the USDF, acting as first assault force, rapid deployment raids and first wave in planetary sieges to secure a bridge point. ODT are often seen as the elite troops within the USDF - Rightfully. Their job is extremely dangerous and their causality rate is the highest among all troops in the USDF, owing to their high risk assignments and the fact they get stuffed into pods and shot from low orbit unto a planetary surface or stuffed into heavy transports that are often shot out of the sky.
"Nothing can beat the human spirit."
The CA is the planet-bound armed personnel, which is primarily concerned with securing colonies and fringes of SolGov in danger of ground invasion. The CA is also the second wave during a planetary siege. Thus, they excel in protracted front line combat and utilize the widest range of combined arms tactics - Mech suits, armored vehicles and infantry assaults. The Colonial Army is the second largest military branch by sheer assets and are found wherever humanity needs the good old boots on the ground and are the archetypal "grunt" force.
The First Rifles of Sol and the Shadow Recon Unit
"With the Fist of Terra, we stand against all."
"Last thing you will never see."
Associated with the CA are the FRS and the SRU, one of the most prestigious military outfits a soldier can belong to. The FRS is often described as an "military within a military" as they possess all assets needed for a small military force on its own - In fact, they even crew a fleet in orbit of Luna. The FRS is the elite honor guard of the Colonial Assembly and solely tasked of protecting the cradle of humanity - Earth and its solar system.
The SRU meanwhile is the elite unit of recon soldiers, drilled for a plethora of high urgency missions and operations deep within enemy lines. Their training is the hardest humanity has to offer for their soldiers and each accepted recruit will end with at least one medal on their chest... And retirement in their early forties.
"The truth shall make you free."
While not a military branch and not officially part of the USDF, the Office of Naval Intelligence (also known as ONI) closely works together with the military. Their primary concern is the gathering, analysis, and interpretation of intelligence for the benefit of SolGov. Their methods and missions often cause strife among the colony population, as many conspiracy theories revolve around this cryptic organization, including digital mass surveillance, mimetic suggestions in innocent citizens and subversion of AI units for the purpose of making unwitting agents. De facto, every force in the USDF has dealt with an agent of the ONI directly or indirectly. And sometimes, they aren't even aware of it.
The USDF has been tried and has fought its fair share of wars over its years. Several of these radically changed how humans approach warfare as well, especially with xenos. The most important conflicts and campaigns are listed here