According to the Belgian embassy, “US citizens in possession of a valid US passport do not need a visa for airport transit, tourist or business trips for stays up to 90 days per 180 days. Do note that your passport must have at least six months validity after your date of entry into Europe (Schengen Zone).”
In other words, you can come into a different part of Europe with 6 months left on your passport and still get into Belgium with less than 6 months because it belongs to said Schengen Zone.