I was offered the bonus if I would continue employment with the company. I didn't ask if it would be sent to me, I just got the idea it would be withheld. The terms of your employment with the company could have been established in various ways. Of course, it is best if it was in writing. If a written contract or personnel policy says how the bonus is computed and when it vests, then your right to the bonus might be easy to prove.
Oral contracts are valid, just harder to prove if they are disputed.
If your boss admits to verbally indicating you could collect the bonus or partial bonus even if you left the company within a year, that verbal promise is just as enforceable as a written contract. Even if you cannot establish a contract claim, you may have a discrimination claim if your employer used an illegal reason for treating you differently. Race, sex and age over 40 are well-known protected classes of employees. Those who report violations of the law may also be protected from retaliatory discrimination. The statute of limitations for breach of an oral contract is 6 years.