My husband and I are doing a 23-day European trip in May 2020. We will be traveling through Italy, Germany, Netherlands & France. After Munich, we have a 4-day stay in the Netherland region. Would Brussels be a good central location if we want to do a day trip to Amsterdam and a day trip to explore the Battle of the Bulge sites?
You have a huge amount of research to do. While it is technically possible to visit places in Belgium and in The Netherlands from Brussels, I don't think it's realistic to make two hour (and longer) trips each way from a "home base". (And I hate changing hotels!) Many battle sites are rural, and are better done by rental car or paid guide with transportation.
How are you getting from Munich to Brussels? If it's not by plane, then you need to apply similar study to the other jumps on your ambitious itinerary. I've been to Amsterdam at least four times, never for less than three days, and to Antwerp (which I like better than Brussels) for a total of six weeks.
If you want additional advice, I suggest that you post a list of the cities you are definitely visiting, and maybe the next tier of visits in parentheses or something, to show that you are flexible about them.
Have you visited our host's European travel advice site, blue menu top left? He even has train maps "in hours" rather than miles, which may be helpful to you.
I have visited the Battle of the Bulge area twice. It's a large battle area from north to south. Well worth seeing! However, even if all you wanted to see was Bastogne, I can't see it being adequately done in a day trip from Brussels.
I agree with the previous post that Battle of the Bulge sites are difficult as a day trip from Brussels. Also, many of the battle sites are in the countryside so it's best to rent a car or hire a local guide to get to them. If taking public transportation, there is no direct train from Brussels to Bastogne. You'll have to transfer to a bus in Libramont. The whole journey will take about 3 1/2 hours. If you're looking for a place to stay in Bastogne, consider Hotel Wagon Leo which is right in the heart of downtown ( see http://www.wagon-leo.com/en/)
The new museum in Bastogne is excellent, so this is worth the trip if you are at all interested in the Battle of the Bulge.