It earns on its own, forever. Spend it on soldiers, or on mining upgrades to earn faster.
Every five soldiers walk out together as a squad you can move straight away. Keep building and it grows to ten, then fifteen.
A squad sent to attack cannot be recalled until its target falls, so pick the moment rather than the target.
Guarded, and worth +2 gold/sec forever to whoever lands the kill. They never heal, so damage sticks — and in team mode you can click an ally's base to donate a soldier.