The Britannian Military includes all the Armed Forces of the Holy Empire of Britannia. It is believed to be one of the largest, most powerful, and most technologically advanced military in the world. Members of the military play an important role throughout the series.
Main article: Britannian Soldier
The use of infantry within the Empire is fairly unknown, and only a few instances of them being deployed are shown, usually when a Knightmare would be too large to use. When they are, squads of Honorary Britannians and Britannians are likely to be used, as "grunt work", like hunting down rebels in a ghetto. Britannian foot soldiers are heavily armed and armoured with helmeted electronic visors.
When they're not in their armor, army officers and soldiers are seen to be wearing blue uniforms.
Main article: Royal Guard
The Royal guard is a personal group of soldiers that serves the members of the Imperial Family, and it has also been shown that the Knights of the Round also have their own Royal guards. However the full aspect of the Royal Guard is unknown.
Lelouch's Royal Guard are presented in more detail: all members are dressed in white-and-gray uniforms vaguely similar to the Knights of the Round with black tricorn hats and grey half masks, the jacket and hat emblazoned in golden eye symbols similar to the ones on Lelouch's Imperial outfit. The tricorns also have drop-down visors similar to the one on Zero's helmet. Some members can be seen wearing a cape, implying they're commanding officers of the Guard. All officers are forced into loyalty as a result of Lelouch's Geass, and act as his personal enforcers. Jeremiah also comments that he subjugated a former noble, implying he is the leader. Most of them are present during the battle of Mt. Fuji as well as the parade during the Zero Requiem.
Knightmare Frames are the most basic type of unit used by Britannian forces, originally developed by the Britannians themselves. Essentially a large armored robotic frame, Knightmares have proven themselves effective in almost all combat situations, and the other world powers have only recently began building their own versions of the Britannian models in order to compete, however often with limited success.
Originally deployed during the invasion of Japan, Knightmares proved that, despite their bulky appearance and high manufacture cost, their agility, maneuverability and adaptability gave them an advantage over simpler, more conventional siege weaponry. By 2017, the Sutherland became the main Knightmare Frame unit of their military. Though it was superior to the Glasgows, it offered little improvements compared to its predecessor. After the introduction of the Lancelot, rapid development of 7th generation Knightmare Frames has been spearheaded by Britannia, with the introduction of their first mass-production seventh generation frame, Vincent. By 2018, though their Knighmare forces are still comprised of Gloucesters and Sutherlands, they are slowly being replaced by Vincents and Vincent Wards.
Main article: Vehicles
Conventional vehicles like tanks, armored personnel carriers and other military vehicles are sometimes used by the Britannian military. Large mobile bases, such as G-1 Bases are often deployed as a command center and field hospital in potentially hostile territory. Similar machines can be seen throughout the series being used to quickly deploy large numbers of Knightmares.
Main article: Britannian Air Force
Actual airplanes are rarely, if never, used in combat by the Empire, though VTOL gunships and Knightmare VTOL are often seen, the latter used as Knightmare drop ships. Since the development of efficient float systems, large hover ships like the Avalon are beginning to dominate the Imperial skies. They serve much the same role landcruisers do on the ground, though still heavily-defended and capable of reaching considerable speeds. Similar float systems are now being equipped onto Knightmare frames, rendering them capable of the same role that traditional combat aircraft would have served pre-invasion of Japan.
Main article: Britannian Navy
While battleships are seen used by the Empire in early battles, ships do not seems to play an important military role in the Empire, usually delegated the role of transport and trade. This is possibly why the Black Knights chose a submarine as one of their first headquarters. However, the Britannian military is by no means incapable of aquatic or even deep-sea combat, as shown by effectiveness of the Royal Marine Infantry (RMI)-13 Portman series of amphibious frames.
"Knights" as by their name sake are elite specialized bodyguards or units of soldiers, usually Knightmare pilots, who operate outside standard Britannian military hierarchy. It is known that there are several Knight Orders in Britannia. Many are under personal command of a member of the Royal family. They are known to wield swords as well as fire arms.
The Knights of the Round are considered the greatest of knights in Britannia answerable to only the Emperor himself.
Here are the list of known Britannian knight orders: