Always leave the doors open.
Consider the future uncertain. I have already told you about how in capitalism changes happen, most of the time out of your control.
You can only control what you think, what you do. You have the right to explore, to change jobs. Don't lock your doors.
No one should be offended if you want to get better. It goes without saying that you come to the United States to do this, to improve. Many times, if necessary for both parties, you can return to a previous job.
Allow plenty of time for the company you're leaving to adjust to the changes your departure causes. It is standard to give two weeks written notice. Your new employer will understand... And if he doesn't, be careful: you start out bad.
Avoid talking publicly about the company you leave even if you are angry and that's the reason for your leaving. You don't leave a good impression in front of anyone, and if you go further and shout too much, the best you can do is to make a souvenir that will give you a smile in the future, but in practical terms, nothing. You will go through a grudge. Your employer may feel that, regardless of how things have gone, your employer has done you a favor by giving you a job, so they will see you as ungrateful.
So don't forget to say thank you before you walk out the door. After a short while, few people will remember you, but if you come back, they may pick you up just for remembering such a simple gesture.
Courteous doesn't take away the brave.