I currently work fully remotely as a full-time employee for a US company. The office is closed with no plans for anyone to return until at least September. I was thinking of working from home in a different country (within the same timezone) for ~2-3 weeks. I would go to this country 'on vacation' (from a customs/immigration perspective) and then would WFH there for the duration. I intend to tell my company this plan; do they have any grounds to tell me no?
At-will employers can fire you for almost any reason or no reason at all, aside from a few protected reasons for termination (defined by things like gender, race, religion, disability, etc.). "Employees who want to work remotely from another country" is not a protected class of individuals, so the company could almost certainly fire you for this with no repercussions whatsoever. Whether they will or not is an different question that's entirely dependent on your specific situation, but in general, US at-will employers have a very wide latitude to "tell you no" by simply firing you.
All you can do is ask your manager. If they say no, then the answer is no. They do not require any "grounds" or justification for their decision.