Imagine that there is a massive military alliance led by China whose sole purpose is to "curtail" the US. Despite several attempts at diplomacy from the US this military alliance has never been anything but belligerent. In the 90's it bombed... The UK? (What's a good US equivalent to Yugoslavia?)

Since then the US has reformed itself to be more like China in part due to assurances that the military alliance would not expand further.

After this the military alliance has expanded to include most of South America and Central America. Mexico is left as the only buffer.
Some time ago there was a coup in Mexico. Wikileaks managed to capture phone conversations between Chinese apparatchiks discussing who should be put in power, and the people they discussed DID come into power, how about that?*

Now the president of America is stating publicly that if Mexico looks to join ~~NATO~~ the anti-US military alliance that has orchestrated coups in both the US and Mexico, then the US will invade Mexico as a precaution. The president states that this is a red line. The anti-US alliance refuses discussions and instead keeps stating publicly that Mexico will be made a member of the anti-US alliance, it has to be in order to prepare for "future conflict". Around this time the anti-US alliance establishes military bases on Cuba. Mexico keeps saying it will look to join NATO. No one is willing to have talks with the US (funnily enough the leader of China actually said in the 90's that expanding the anti-US alliance would be a mistake and would lead to war with the US).

While all this is happening mexican fascist militias are bombing regions of Mexico with majorities of ethnic USians. This is in breach of two treaties, which is wholly disregarded by both Mexico and the anti-US alliance.
It looks like the red line is about to be crossed. A red line that the anti-us alliance had pledged not to cross.

Now if all that had happened, can you understand why the US might invade Mexico?

Okay then, let's leave this imaginary scenario and... wow would you look at that! It all has parallels to modern day Russia-Ukraine. It seems like this heavy-handed analogy that you're going to dismiss due to being "too long" or "unrealistic" (because let's be honest, you're the type to struggle with hypothetical) actually had a point! Wow pulling your head out of your ass sure can be fun, huh?

*The coup was led by a fascist militia whose main motivation was to get ethnic USians out of Mexico. After the coup a public referendum was held