I was terminated for having a minor accident driving a piece of equipment I was not formally trained to operate. Is this legal?

Barring a contract speciifying a specific term of employment (e.g. a 3 year employment agreement) or limiting the reasons why an employee can be fired (e.g. employee can only be terminated for good cause etc.), an employee can be fired at any time for any reason except those specifically prohibited by statute (e.g. race, religion, age, some types of whistleblowing, etc.)