It's a false statement that US soldiers are not involved in this conflict:
It's not secret that there is some US troops fighting in Ukraine. Same like troops from Poland, Georgia, Canada and even Russia and Belarus. But they're not part of regular army, it's veterans, volunteers or so called foreign legions
Are you sure you know the origins of the conflict?
In late 2013, Ukrainian President Yanukovych suspended negotiations with the EU in favor of strengthening relations with Russia.
After that, mass protests began in Kyiv.
In February 2014, Yanukovych was ousted from the presidency.
In April 2014, the residents of Donetsk and Lugansk declared their independence.
In response, Acting President in Ukraine Aleksandr Turchynov announced the start of an "anti-terrorist operation" in eastern Ukraine.
You can read a brief background of the conflict, at least in the
Britannica.
I remember origins of conflict very well. Independence of Donbass and Luhansk, recognized by no one, backed just by Russia with their army in region. It's same like referendum in Crimea.