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)