I'm not sure of the time frame here but if Belgium is an option and you are talking about WW1 I would go with Belgium because, in the beginning of WW1 The German Army decided to push thru Belgium to finish off France quickly so they could then focus on fighting Russia. Britain was not ok with this and joined the war here and helped the French and Belgians fight off germany albeit slowly due to German entrenchments.
For WW2 I think it would be Belgium, Netherlands, and Luxembourg. Other neutral nations may include: Poland, Czechoslovakia, Denmark, and Norway. Most of these nations joined what is called The Allies when they were attacked so they were no longer neutral once under siege.
If you have any details so I can help more let me know.
