One of the more interesting neutral supply centers in 1901 is Belgium. There are four possible outcomes in fall 1901 for Belgium. It can be conquered by one of the northern powers, or it will remain neutral. Obviously the situation in Belgium cannot be looked at without taking the complete northern triangle into account. So, that is what I will do in this article.
Most English players see Belgium is a nice-to-have supply center, rather than a necessity. Norway is what you need to conquer; If you are looking for a quick exit of your French colleague, I beg to differ.
In case England is looking for a conflict with France (or even Germany) early on, getting an army into Belgium provides you with an extremely strong position. However, it will be difficult to conquer Belgium on your own, especially if you need your northern fleet to occupy Norway in fall 1901.
In case of an England-Germany alliance, the German player could be asked to support you into Belgium, which he might be willing to do if you have show your intentions from the start.
If you are interested in Belgium, it could be wise to try and occupy the Channel in Spring 1901. Although this might bring two fleets in position (NTH and ENG) I do not think it is necessarily the best solution, mainly because you would still want to move NTH to Norway in the fall. The main reason for occupying the Channel is to force France to defend his home supply center in Brest, which in return means he is probably not making a move for Belgium.
As an English player interested in Belgium, you should try and persuade Germany to move his fleet to Denmark in Spring 1901. First of all, this means he cannot use his F (Hol) for a support move into Belgium in the fall, and secondly, Russia will feel threatened by the possible bounce in Sweden, meaning even if Russia has made a move from Moscow to StP, you still have a shot at Norway if you play your cards well.
Another option is to not get yourself involved in Scandinavia. This is tricky, but possible. You need to secure a good deal with the Russian player, asking him to only move an army to Norway, and let each other be after that. However, most Russian players will fall for the temptation in 2-3 years and will want to move beyond Norway, so be careful if you make this deal to get some safeguards.
Germany is the most likely country to occupy Belgium, if he really wants to. The right opening for a move on Belgium is F (Kiel) – Hol and A (Mun) – Ruh (or Burgundy, if you can make it there).
There are two elements to consider when making this opening. First, you have to live with the inability to influence the situation in Scandinavia. Russia will conquer Sweden in Fall 1901, and your army in Denmark does not give you many options to make a strong impression in the near future either. Secondly, the supply centers in Hol/Bel/Den mean you have three builds. Depending on your strategy this is a good or a bad thing. I do believe Germany is one of the few countries that could explain he needs three builds due to his center position. But some of your neighbors might find your growth excessive and decide to team up against you. Excessive growth early can be very dangerous if not explained properly to other players.
I believe Germany could also profit from a situation where Belgium remains neutral in 1901, and seize it in 1902 as other countries grow to six supply centers as well.
Playing France, you really have the neutral supply centers out there for you. Two are in the south, which are not threatened, and Belgium in the north. Making a move for Belgium could be quite smart when playing France, as it will guarantee you a firm seat on the table in the northern triangle. You could dedicate two units to this, and let your third unit conquer one supply center in Iberia. An unconventional opening could be moving to Burgundy and Picardy with your armies, and your fleet to MAO. The risk is that England moves to the Channel (which requires you to cover Brest in the fall) or that Germany bounces you in Burgundy.
Another option is to convince Germany or England you need support into Belgium in 1901, this is not such a likely option to work out for you. A Germany player will have difficulties understanding why you should get Belgium instead of him, and England is focused north, not looking at Belgium, or out there looking for Belgium himself. Only a good proposal towards England, where you will support him to Holland in 1902, could secure your support.
Belgium is the key neutral supply center early in the norther triangle. Your play around Belgium will certainly be a part of your overall negotiation. There are lots of examples where you do not take Belgium but can be the diplomatic winner making a key ally (or enemy) for the rest of the game. Do not be greedy, take Belgium if you can, but please remember that making friends on the board could be at least as important.