Matthew Livelsberger, the Army Special Forces soldier who blew up his Tesla Cybertruck outside the Trump Hotel in Vegas, was battling PTSD -- and now we know his motive.

The Clark County Nevada Sheriff's Dept. just released a portion of a letter Livelsberger wrote, saying ... "This was not a terrorist attack, it was a wake up call. Americans only pay attention to spectacles and violence. What better way to get my point across than a stunt with fireworks and explosives?"

He goes on ... "Why did I personally do it now? I needed to cleanse my mind of the brothers I've lost and relieve myself of the burden of the lives I took."

What's interesting is Livelsberger was a Donald Trump supporter, so the question ... why did he use an Elon Musk vehicle to make his point in from of Trump's Vegas hotel?

Livelsberger wrote ... "Fellow service members, Veterans and all Americans, TIME TO WAKE UP! We are being led by weak and feckless leadership who only serve to enrich themselves."

He cautions, "right now we are eternally ill and headed towards collapse."

Livelsberger was a Green Beret in the Army and served in Afghanistan and Central Asia.