I play with your teeth in the mouth

I play with your teeth in the mouth. Before I enter your mouth, I'm hard. But when I come out of your mouth,
I am soft. I came from America.
Now I am also made in Korea.
I came to Korea before the Korean war. I can get rid of bad smell in your mouth. I'm something to chew.
I'm made of sweetened and flavored chicle.



what am I ?