Total War: THREE KINGDOMS

So I tried this achievement several times a while back but had no luck and took a long break. However, I just learned that this game will be leaving game pass soon, so I'm going to leave some of the things I learned from my previous attempts for people that want to try this before it's gone. If I manage to complete it before the game leaves, I will update this guide with anything else I learned. While strictly speaking, this is not the most difficult achievement, it is in my opinion the most frustrating. Good luck everybody.

Edit: This guide is longer than I thought, so I'm going to try a TLDR list first and then list the longer explanations after that for people that want to read them.

Edit 2: FINALLY DONE! I'm fairly certain the diplomacy cheese I tried is what screwed me last time. Zheng Jiang is probably the best pick for non-dlc players, but I would check out Yellow Turbans as well if you have them, since they already have limited diplomacy options to begin with. Start by taking out Zhang Yan and use his starting base in the upper mountain as your starting base. From here, you can either expand straight eastwards, taking out Gongsun Zan and capturing all the northwest territories, or just stick where you are. No matter who you pick, don't expand too quickly in the beginning. Focus on finding and building up a good starting area. Once you can field 2-3 armies is a good time to slowly expand. The last thing I noticed is that about halfway through, I noticed a looter faction in the top-left that I'm pretty sure has been sitting around for a few turns before I properly noticed them. My current theory is that they don't count because they will never show up in the faction list or diplomacy window, however there may be other rules going on here.

TLDR

• Don't do legendary, save often
• Don't try to make a deal, just declare war immediately
• Use Faction List from flag icon in bottom right of UI, some factions may show on map but not Faction List (Liu Chong), order your army to attack them to declare war on them
• Very long game because you can't use vassalization/annexation
• Starting location matters a lot more
• Don't play Han factions, choose Bandit, Yellow Turban, or Nanman

Full Explanations

• Don't try to combine this with the legendary difficulty achievement. From what I can tell the game checks at the end of each turn, so you'll want to save frequently and rollback in case you forget to declare war on a faction before ending the turn.
• One exception to the above seems to be making deals. I tried a run where I stole as much money/territory through diplomacy as possible from factions before declaring war on them and didn't get the achievement. It's possible I missed a faction in one of the turns, but I don't think so and I'm not going to ruin another run just to test that theory. Obviously this means any factions with diplomacy-based mechanics are useless unless you really like their unique units.
• You can use the Faction List to see at a glance what factions you are at war with currently. It's the flag icon on the bottom right UI above the minimap and I think it's the same list as in the diplomacy menu. However, some factions will be shown on the map and not the faction list, such as Liu Chong at the very beginning. Some times it happens vice versa too. I'm not sure if the game checks both or just one or the other. To be safe, you can declare war on any faction on the map by selecting one of your armies and ordering them to attack that faction's territory or army.
• Because diplomacy is virtually nonexistent, the only way to get territory is through battle. This may seem obvious, but I want to emphasize this because this means it'll take significantly longer to get to the win condition than normal because you can't use annex/vassalize to pad out your territory count so expect a very long endurance match. Also, your entire income will come from structures and battles, which historically for me are the smallest sources especially early on. So if you're like me, be prepared to adjust your development plan and have a slower start than normal.
• Starting location matters a lot more for this specific achievement than others. You want to be at least on the edge, and as close to a corner of the map as possible. This will limit the directions you can be attacked from and make it easier to both defend and expand from. That said, you can use the geography to your advantage. The bandit starting locations in the north, for example, have mountains to the west, which act as a natural barrier. Most of the south is controlled by the Han Empire, which is usually not an aggressive faction, so you can sort of consider them a temporary barrier as well. I've also seen some people pick a more inland faction and relocate somewhere else with their starting army, you can try that too if you absolutely need to play a specific faction.
• Han factions are bound to imperial court mechanics, which apply debuffs at low imperial favor (to corruption, satisfaction, and diplomatic opinion iirc). If extremely low, there will occasionally be events where the emperor applies another debuff to try to curb your "aggressiveness" and some of them feel crippling, like lower income and higher army upkeep iirc. If you get declared an enemy of the state, there's an event with 4 options. 3 of them claw back imperial favor, and the last one says to break off from the empire but it doesn't actually work that way, it just gives you some military buffs. The issue with managing all of this is that you automatically lose imperial favor with every battle in which you are the attacker, including battles where the garrison sallies out to break the siege (I guess the emperor prefers the city to starve to death instead). So the only way to avoid them is to play extremely passively, limit the amount of battles initiated and try to bait the enemy into attacking you instead. Encampment stance helps a little bit, but in my experience, even on the easiest difficulty the AI won't attack without a clear advantage, so you're always on the back foot this way. Instead of dealing with all that, I'd play somebody else instead. Without dlc, you're only options are Bandits. I had luck with Zhang Yan, but I think Zheng Jiang has a better synergy for this achievement. If you have the related dlc, feel free to try Yellow Turbans or Nanman.