The aim of this guide is to turn this achievement into a normal playthrough with a twist instead of a grinding mess of failing, scouring the internet for information, and ending up more confused than when you started.

You can form the federation when the total population of you and your ally gets above 50% of the total population.

Total population consists of:

• Allied population - green meter on the screenshot, you will increase it as much as you can

• Non-allied societies - gray meter on the screenshot, you will not increase it at all, it will not increase by itself

• Rival population - red meter on the screenshot, it will keep increasing and cannot be influenced, you can take it as the rival plays on an entirely different map. The goal is to get your only ally to surpass this increase

So how to reach this goal:

Basically all you need to do is to build all six of the population-increasing buildings for +11 per day (Modest Tents, Greenhouse, Kitchen, Nice Housing, Rental Housing, Basements) for your chosen ally and get to the alliance only by trading goods that increase relations.

You must not build any population-increasing buildings in the other settlements, as that would constantly move you away from the required 50% of the total in addition to the rival. You can, however, build Taverns for income and Trading Posts for faster trading.

Then it's only a matter of time. Enjoy the game.

Some additional points:

• You don't need to rush too much, set up your economy first. Once the ally's population growth surpasses the rival, you've won, so don't stress too much.

• You can, however, rush discovering all the societies so you can choose the one that provides you with a good benefit, you will be allies for a long portion of the game. Especially postpone scavenging for meds, as it takes a lot of time from your specialists.

• I'm not sure how or if difficulty setting influences the rival's population gain. I finally got it on all the easy options, and it went only from 750 to 950 in 140 days. I found my older save from an earlier try on higher difficulty, and it went up to 933 in 211 days (the same playthrough had 715 on day 67). I did a quick test run on the hardest possible setting on the normal game mode, and the rival has a population of 667 on day 9. So the starting number of the rival's population gets lower the harder the difficulty. Based on these numbers, I would assume the rival's gain is not going up the harder you set the game.

• You can ignore requests (meaning not resolving them in the request screen) for the whole game, once you get to the 1000 relation with your chosen society, resolve one (ideally one that reduces someone else's population when not fulfilled) to have room for receiving an alliance proposition.

• And of course, set up a few good survivor outposts to visit them later for bumping up your population when you need it. You do not need to go overboard with it, as the idea is the ally will do most of the multiplying.

• I'm not sure if it works in game types other than normal game, feel free to test it and let us know in the comments. I also had the two other DLCs turned off.