Why will this happen in the first place?

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It's year 2051, I had to travel through time to prevent this to happen,
I was born on the date August 12, 2002, in Irvine, CA, in a middle class family.
US was declining fast, and the process went accelerated after 2028, when a trans feminist communist fucking SCUM becomes president, the US military becomes small and weak under them, and the aztlan nationalism (backed by multiple latin countries) was on the rise.
The conflict started in the year 2039 when reconquista nationalists murdered 12 border patrols in California, this soon turned in to civil wars and race riots in the US Southwest.
In the year 2042, the year which I will never forget. I was Removed from my beloved homeland, the american southwest, and had to go to Colorado, where is relatively less dangerous compared to the former border states.
In the early summer of 2043, the latin powers and the mexican reconquista government declared war on the US.
CA, AZ, NM, TX, and OK immediately became warzones.
It's early 2051, US had somewhat pushed off the further invasions from the south, but were still struggling in the former border states.
but why?