After proving that he is too smart to be a mere soldier, Ender is given command of an army for himself. It is designated Dragon Army, a name that carries many negative connotations:
"There hasn't been a Dragon Army in four years... No Dragon Army in the history of the Battle School ever won even a third of its games. It got to be a joke."
(Card, Ender's Game, Google Books)
His army is composed largely of new recruits, so he has to work extra hard to train them; all the other armies have been trained for months already. In addition, his training time has been curtailed by the Battle School, which wants to test his resolve and ingenuity; he has only three weeks to train for his first battle, and after that, he has battles almost every day with unfair advantages given to his opponents. Ender prevails, however, and his Dragon Army wins all of their battles through superior tactics and military thinking.
Ender's entire army is green, meaning it is made up of completely or almost completely untrained soldiers. Many of them are fresh out of launch groups, meaning Ender has to train them all himself. Also, instead of the standard few months to train, Dragon army is given only 3 weeks.