llets spell this out for the US. Flip roles here and see how you react.
US and Mexico typically have good economic relations at least compared to others. Mexico typically doesnt do as much business outside of the US as they do with the US. The US has also on multiple occasions conquered and or fought Mexico and has even taken territory from them.
Now Mexico doesnt like their reliance on the US, and elects a series of pro Chinese leaders for about 30 years. Now eventually one pro Chinese politicians who has been pro Chinese on campaign and for more than a decade gets elected. After announcing a big treaty with China they actually end up signing a deal with the US instead, and back out of their Chinese deal. Now the Mexicans dont like that they elected this guy because he was pro Chinese, and got pro American treaties out of it. They protest the guy and some protestors get shot. Not unheard of in Mexico. The now pro American leader gets heli-vacced out by Seal Team 6 to a prepared mansion and gets a 7 figure allowance from the US government. Meanwhile the Chinese ambassador to Mexico is seen a couple days before interacting with the protestors and an old Chinese politician makes some anti Mexican leader comments back in china.
Next, some little green men occupy Baja California. Kick out the local officials. And then hold an election where they stand over the ballot boxes with guns while masked. And America Annexes them.
As that is going on a "bunch of locals" supported by some very vocal Americans, in person and financially, in next door Sonora decide that despite being a clear majority Mexico they are going to leave Mexico to be independent. In their local mines they just so happen to find some fairly modern Abrams not commercially available. They also find a bunch of new American service rifles that the entire US military doesnt even have yet, as well as a bunch of other current edition US military equipment. More modern than what Mexico itself has. Then armed with all this American stuff, they round up all the Pro Mexico , pro Chinese, people and deal with them in various methods, including having some selectively chose to go to Guantamo Bay.
Sometime after that the breakaway region and mexico start fighting. Let's just say that Mexico fires first. Things arent going well for Sonora, and then the straight up American military shows up to help fight Mexico. And the breakaway region suddenly isnt about to collapse bringing peace to the area. As well as a mass deployment by American military along the entire Mexican border, and America gets allied Guatemala to also saber rattle at Mexico allowing American troops to take up that border as well.
Now in that situation, would you believe, Mexico, China, or the United States was at fault?