National Geographic Challenge!

There are 36 territories in total, so you'll need 18 territories to get this achievement. Because of that, you'll want to play this with only two players, because you win at 2200 points (and can therefore claim 22 territories before the game ends).

With four players, you win at 1700 points and because of that it's impossible to get this achievement.

The basic strategy is to only win 100-point territories. That's it. With player two, I would first look to see if player one started out with any territories worth more than 100 points and win those first. Then spend the rest of the game moving back and forth between territories already owned by player 2. That will prevent any questions from being asked on player 2 and will make this last a lot less time.

Player 1 should then move from 100-point territory to 100-point territory, claiming them. You should be able to get to 18 countries with room to spare. The achievement will pop as soon as you get the 18th.

You can get this and "Diversity" in one playthrough -- just make sure you capture a territory in each region while you're doing this.