Commonwealth

Allied Kingdom (連合王国) is a powerful island nation with many colonies in the alternate world. Its capital city is Londonium. It stayed neutral at the beginning of Great War for worrying about whether the Republic will defeat the Empire and violate the balance of power. However, it was searching the chance to join the war after the Republic was pushed back by Imperial Army, and finally did it when the Empire rejected its requirement for “restitutio in integrum”.

International Relations
Unified States (合州国) - They have special relationship of colonial history in the past. After Allied Kingdom waged war on the Empire, it receives the volunteer mage units from the US and buys many heavy weapons from the US arms companies.

Francois Republic (フランソワ共和国) - Before the Great War, they have not good relationship due to the history in the past. Allied Kingdom stayed neutral at the beginning of Great War for worrying about whether the Republic will defeat the Empire and violate the balance of power. After the Empire took advance in Great War, they began to share the intelligence in secret and send the volunteer forces. After the fall of the mainland of Republic, they formed an alliance to fight the Empire.

Rus' Union / Russy Federation (anime) (ルーシー連邦) - Because of the different ideology, they treated each other hostile before the Union declared war on the Empire. However, after the first invading army of the Union was defeated, they started having diplomatic exchanges, such as exchanging the arrested spies, enabling Arctic convoys, sending expeditionary force, weapon and material supply.

Army
The Allied Kingdom has a third level of advanced military with limited trained soldiers. They are still no more much to the Empire's forces. When Operation Revolving Door was performing, they only had 7 infantry divisions and 1 cavalry division on main islands to send abroad, which was just a drop in the bucket compared to the Empire.

Navy
They have large amount of heavy and long-ranged battleships, containing some super-dreadnoughts. They have the more advanced and powerful mechanized ships than the Empire. Their tactics of naval mages are the most advanced, such as taking off from submarine, ship-borne attack, and short-range air support against the marine corps.

Airforce
They have strong air power and resistance to fight with the Imperial air forces over Dodo Bird Channel. All of their pilots are elite and highly-trained personnel. Therefore, they put a lot of effort to maintain the air superiority in the sky.

Spy System
Allied Kingdom is one of the few countries who used knowledge, deception and gathering information about the enemies. They even broke high-level encrypted radio and teleprinter communications of the Empire, which had a special code named Ultra.

Trivia

 * On the maps that show up in the background of the anime, the country's name shows up as "Commonwealth" in a highly stylized font.
 * In the manga, the full name of Allied Kingdom is Allied Kingdom of Albion.