Greetings!
In the last DD I touched upon becoming a Tributary, and its associated penalties/advantages. In this DD I’d like to talk about China and their various interests on our side of the map - which more than often includes having Tributaries!
China is mostly content staying within the Middle Kingdom itself, but it is not unheard of for them to want Tributaries outside of it. While only an Expansionist China will actively pursue more and more Tributary States, even a mellow China will want to have a few of them. China will, at times, go to war in order to make someone their tributary state - they will often target small realms in the Tarim Basin or Tibet areas, though small states often tend to give in before an actual war is declared.
Even if China has land in the west (the Western Protectorate) they often bring in troops from China. These troops usually arrive in the Anxi region just above Tibet. China, unless Expansionist, does not want to overextend while warring in the west, so they tend to not call upon more troops than they think they need to defeat their target. It’s not uncommon for them to be wrong though, most rulers will be able to defeat China if they outmaneuver them, hire enough mercenaries or simply get lucky. Forcing China into a surrender is a very significant thing to do, the spoils of such a treaty are often way more than enough to recover from the losses.
The power of China rises and wanes with their Status and with time, sometimes they will be weak and sometimes they will be unstoppable. A Stable China will be able to match their enemies should they attack or be attacked, while a China suffering from a Famine or Plague isn’t nearly as strong. If the Western Protectorate has established itself on the map, a good time to strike at them would be during such a status, it is also a good idea if they are suffering Unrest or are in the middle of a Civil War. During times such as those, subjugated realms tend to break free without much opposition from China.
On the other hand - if China is in a Golden Age it would be most unwise to meddle with them. They will be able to summon both more and better troops, and they tend to want larger realms as their tributaries - even if they don’t go after more of them than they would normally. While a China that’s in a Golden Age is more threatening, it’s also more profitable to ask them for Boons as their effects will be much stronger and last for longer.
If China is Expansionist, they will want to secure land for themselves. It is fairly rare for China to be Expansionist. It is the most common when/if they are ruled by the Mongols or other Altaic Emperors. When China is expansionist, it’s not necessarily stronger than it is normally - but it will go after bigger fish. They will target almost any realm, despite size, and try to either force them to pay Tribute or more commonly to subjugate them. Should China lose a war while expansionist, it might be thrown out of Expansionism and into Unrest. Should China be both Expansionist AND in a Golden Age, they will be an unstoppable juggernaut - this is an exceedingly rare combination which at most happen one in every ten games or so.
Worth noting is that China tends to keep to the same area when they go to war. They might, for example, want to make everyone in the Tarim Basin tributaries or subjugate northeastern India. They tend to not divide their interests unless they have to.
It is not only the Emperor who might go to war in the west - adventurers will at time spill out of China in order to make a new home for themselves. There are three major archetypes:
Sinicised Altaic Tribes - The leader of a Taoist Jurchen tribe might arrive along the Silk Road or via the Steppe to make a new home for themselves in the far west. These act much like normal raiding adventurers, though we’ve revamped the raiding adventurer system to be more interactive for the player, with adventurers less often outright attacking you - instead they prefer to haggle, potentially offering you plenty of gold for a province to settle in.
Displaced Princes - After a Civil War is lost, a scion of the former ruling dynasty might escape west. These characters start out with a formidable army and bring a lot of wealth with them. Such characters are very amenable to negotiation, and will often pay you extraordinary sums of wealth to be allowed to settle in your lands.
Rebel Generals - If a Civil War is won, a particularly ambitious general might make his way west to found a new Chinese Empire. They often bring what remains of the rebel armies, making them very dangerous. Having the nerve to found a new Chinese Empire will draw the ire of the ruling dynasty in China, often making them put everything else aside in order to crush the western pretenders.
Note that everything regarding Chinese invasions are fully controllable by Game Rules - you can set China to only take Tributaries and never do land grabs, for example. You can also choose to only have the adventurers, with China never extending outside of the Middle Kingdom, among several other settings.