I am taking an employment law class and "At-Will" states can be tricky.

The groups protected by Title VII of the Civil Rights Act of 1967 are: Race, Color, Religion, Sex (gender actually), or national origin. The Americans with Disabilities Act of 1991 was passed to cover discrimination on the basis of disability. The ADEA or Age Discrimination in Employment Act of 1967 covers those over the age of 40 from being discriminated against from an employer.

So for the main protected groups by Federal law we have: Race, Color, Religion, Sex (gender), national origin, disability, and age (over 40). Pollitical affiliation is not covered by Federal law, sorry.

California law covers discrimination on groups of race, color, national origin, ancestry, sex, religion, age, marital status, sexual orientation or disability. These laws may vary by state. Missouri seems to only cover race, color, religion, national origin, ancestry, sex, disability or age (40-69). Notice that Missouri does not cover sexual orientation or marital status, yet sex is covered and may be applied to pregnate women and other differences between the sexes (the PC term is Gender, not sex).

There also are sexual harassment laws, but this case does not qualify.

So in some cases your manager can act like a jerk and fire you for silly reasons not covered by the above groups. Maybe they don't like your taste in music? Maybe they think you are an obnoixous person and fire you for that? Maybe they caught you with a Dilbert calendar? In an "At-Will" state (I think 28 US States are "At-Will" states) they can fire you for silly reasons not covered by protected groups or no reason whatsoever.