I'm a US citizen and almost 10 years ago I was arrested for a misdemeanor. Although I didn't get convicted I still went through a diversion program and got the arrest expunged. Will I still be able to go abroad to a country like Japan or is that not possible now?

I'm not a legal expert, but in many cases, having a misdemeanor expunged means that it's no longer on your record, which might improve your chances of being granted a visa or entry into a foreign country. I suggest you check with the Japanese embassy to get the most accurate and up-to-date information on their visa application requirements.

If you weren’t convicted you didn’t commit a crime, at least as far as the law is concerned. You say the arrest was "expunged" . Never heard of that, but perhaps it is just a US thing in any case and means removed from your record, I don’t think US citizens need a visa for Japan so it is irrelevant.

There are lots of countries that do require US citizens to apply for visas and applications forms will doubtless ask whether you have any criminal convictions and the amswer is clearly no. Even if you did commit a "misdemeanour " and were found guilty, the majority of countries ignore those after a period .