Usage patterns
〈場所〉に軍勢を集めるto gather an army at a place
He gathered his army around the castle.
軍勢を率いるto lead an army
The general led a large army into battle.
軍勢が押し寄せるfor an army to advance/surge forward
The enemy's army surged toward the border.
〈数〉の軍勢an army of a certain size
An army of ten thousand surrounded the castle.