Harrison Jozefowicz, 25, is an Army veteran who quit his job with the Chicago Police Department and headed overseas.
He talked to ABC's Martha Raddatz, who's covering the war in Ukraine.
Raddatz: "But are you going to be in the fight?"
Harrison: "Uh, me personally, yeah, if it's needed. I'll be in the fight."
Raddatz: "You just quit your job and got on a plane?"
Harrison: "Pretty much."
Harrison: "This is the right thing to do. This is already...has more refugees than Afghanistan. And we can't sit idly and watch it happen. I can say that Putin doesn't know what's coming."
Raddatz: "A tougher fight?"
Harrison: "A very much tougher fight. We're going to see an exponential increase in numbers. Very, very shortly here....They're going to be the hardcore, battle trained Americans, Brits and everybody else in between."
Jozefowicz said he plans to stay in Ukraine as long as he can support his wife back here at home.
The State Department is warning Americans not to travel to Ukraine to fight and President Biden said no U.S. troops will be joining the war.