An army was a military land force.
The Fourth Doctor marched with the Filipino Army in the 51st century on Reykjavik during World War VI. (TV: The Talons of Weng-Chiang)
Morris Heggie was a general in the Union Army during the American Civil War. (PROSE: Blood and Hope)
Sinnott was the General of the Irish Army and ordered them to attack when negotiations with Oliver Cromwell failed. Ace wanted to stop him from fighting. (AUDIO: The Settling)
The Eleventh Doctor assembled an army (mostly composed of Silurians and Judoon) to rescue Amy Pond from Demons Run. (TV: A Good Man Goes to War)
The War Lords attempted to create an army capable of universal conquest by brainwashing humans from various Earth conflicts and recruiting those who survived the War Games. (TV: The War Games) When this failed, the War Lords tried again with a more streamlined approach centred on Nazi Germany. (PROSE: Timewyrm: Exodus)