Risk

To earn this achievement, you need to save up 5 stars worth of territory cards. You earn a territory card every time you conquer and enemy territory (up to 1 per turn), and upon finishing off the last territory of a particular enemy (color), you will receive all territory cards they had in their possession.

Cards are worth either 1 or 2 stars, which is shown at the bottom left of each card. Save up 5 stars worth, and then trade in at least 5 stars worth in a single turn. You can turn is as many cards as you have available on your turn.