An army was a branch of a military responsible for conducting combat operations on land.
It was told in the Vulcan story of Nirak that Nirak was standing watch over the gates of Gol when he mistakenly thought that an approaching army was a sandstorm, and failed to send a warning. The army destroyed the city, but they spared Nirak's life. His name later came to mean "fool" in the Vulcan language. (ENT: "Kir'Shara")