Military rank
During World War II, due to the large scale of combat, multiple corps were combined into field armies commanded in theory by a general (four stars), but often by a lieutenant general (three stars), and comprising as many as 240,000 troops. Armies are numbered by spelled-out numerals or functional titles, using their nationality in "combined" forces (e.g., Eighth United States Army, Third ROK Army, British Army of the Rhine). These were in their turn formed into army groups, these being the largest field organization handled by a single commander in modern warfare. Army groups included between 400,000 and 1,500,000 troops. Army groups received Arabic numeral designations and national designations when combined- e.g. 21st Army Group.

Military ranks and insignia of various nations: 

See also: List of comparative military ranks
Ranks of nobility and peerage
UK and US military ranks compared
Shoulder mark

References: 

Citations: 

Sources: 

External links: United States Military Rank Insignia, Background, & Description
Glossary of Ancient Greek Military Terms
US Military Ranks, Ratings, and Grades by Branch (with insignia)
Military rank
US Military ranks and rank insignia
British Army ranks and responsibilities
Military ranks