Is it ok to leave a job just after you've joined?

Let's say you have an amazing job offer (almost your dream job) and you've been invited to join, but there's a 50% chance that after about 2 months you may receive another invitation to pursue your life goals (which are more way important to you in the long term).

Would you be honest and explain this possibility to the employer upfront? If you were an employer, how would you react to this?