HUMANKIND

An easy way to get this:
- set map to small(optional, will be faster), 10 players (max AI), easy difficulty
- just play the game, invest in food and military tech
- during the game accept treaties from your direct neighbours (trade etc)
- during the game, preferably era 2, find an AI with only 1 or 2 cities/outposts (most likely one of your direct neighbours).
- optional: ask them to open boundaries for your armies in thé treaty screen, this should be accepted if you've been positive towards them
- make some armies and surround thé cities
- order in a surprise war and attack them next turn
- when you captured the cities keep your armies in the territories, so 2-3 turns later you will get the option to "Force Surrender"
- make them a vassal or whatever way you feel like, after that the achievement should pop up

The "6 turns count" starts to count when you declare war on a enemy.