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)

When the First Doctor was a child, he feared that he would not become a Time Lord and would instead be forced to join the Gallifreyan army. (TV: Listen) During the Last Great Time War, the Time Lord army was overseen by the War Council, which was itself led by the General. (TV: The Day of the Doctor)

The first known Dalek army was assembled fifty cycles after the Thal-Dalek battle. Having survived the battle, the Dalek Supreme manufactured hundreds of Dalek War Machine casings and led its revived army against the Thal City. When the First Doctor took control of the ionising towers, the Dalek army retreated and was destroyed when Ian Chesterton overloaded the power feeds of the Dalek City. (AUDIO: Return to Skaro) During the reign of the Dalek Empire, the Daleks began keeping armies in suspended animation. (TV: Planet of the Daleks, WC: The Sentinel of the Fifth Galaxy) In the Last Great Time War, the Daleks began boosting their overworked army, converting conquered humans into Daleks at an accelerated rate to serve as cannon fodder and harvesting versions of themselves from alternate realities to serve as shock troopers, creating the Skaro Degradations. (PROSE: Engines of War) Numbering at a "billion billion" units, the Dalek army attacked Gallifrey on the last day of the war, destroying itself in its own crossfire when the planet disappeared. (TV: The Day of the Doctor) When the New Dalek Paradigm was raised, they used the production facilities of Goth to quickly build a galaxy-conquering army. Historians who studied the Dalek race believed that the New Paradigm had deployed the totality of its army during the Siege of Trenzalore to prevent the return of the Time Lords and as a consequence, its destruction at the hands of Eleventh Doctor had been the final defeat of the Daleks. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)