What qualifications are required to teach undergrads at college level in the United States?
I am not aware of any officially required qualifications for teaching higher education. While often universities "require" supervisors, committee members, and examiners of PhD students to have a PhD, these can often be waived if the person in question holds another comparable degree (e.g., ScD or MFA) or has sufficient experience.

From a practical standpoint, to be competitive for a full time teaching positions in the US, you will likely need a PhD, or the equivalent terminal degree in your field. Individuals without a terminal degree and lots of work experience can sometimes teach individual classes in their area of speciality. Even at community colleges, a PhD is a desirable qualification that improves your chances of getting a job.