Belgium is a English province - That's no bad for France looking for gains to the East (=Germany). France only need is to hold the line in Low countries aiming at Munchen. The Brest Fleet is critical holding England or better facilitating a German-English war in Low countries.
Belgium. To France, the Revolutionary left bank of the Rhine. To Germany, the long-lost Southern Netherlands of the thousand-year First Reich. To England, the dagger pointed at the heart of the beloved home island. In history, the three great nations solved their dispute over Belgium by arranging that it fall under the rule of distant Austria. In Diplomacy, the three great nations have no such option over Belgium. In Diplomacy, the three great nations must fight.
In the mid- and endgames, Belgium bears little special significance. It has only six neighbors, so is not a critical flanking position; it is just another supply center on the way to eighteen. But the basic shape of an entire game of Diplomacy often emerges from the opening, and, in the opening, Belgium is the single most sharply-constested province on the entire map. France, Germany and England all want the early Belgium build very much.
France naturally sees Belgium as France's rightful half of the Low Countries. A Frenchman who can negotiate Belgium in 1901 stands in position to build two fleets and an army at the end of 1901 and begin his conquest of England in 1902. If Italy is well-occupied with Turkey, France may already be on its way to victory.
Belgium is more important to Germany than it is to any other country. Germany wants and needs both of the Low Countries! The German stands right in the middle of the map, doesn't he, and never seems to have enough units to defend his lands. There is a substantial difference between a five-unit early-game Germany and a six-unit early-game Germany. German chances of survival and eventual victory go way up if Germany can somehow negotiate Belgium in 1901.
England sees Belgium as the only available neutral beach to begin its slow invasion of the Continent. The earlier England can secure Belgium, the better. Furthermore, if England fails to negotiate Belgium and France gets it in 1901, instead, then the four-unit England suddenly stands quite vulnerable to a quick six-unit French invasion, doesn't it? Such 1902 invasions may well be the number-one cause of English elimination from Diplomacy.
Both England and Germany would rather see the other of the two get Belgium in 1901 than see France get it. As one of hundreds of actual examples, in the USIN game Rcgm (began May 1996, not yet finished February 1997), I played France and decided to risk going for Belgium in 1901. I told the German that I would peacefully and fairly split the Low Countries with him. I told the Englishman that I would support him into Holland. The result was that Germany opened fleet to Holland and England opened fleet to the Channel! Only a strong Russian ally saved France from early elimination. Belgium is hot!
Are we suggesting that Belgium is too controversial, that a good player leaves this critical province alone? No, not at all, not necessarily. What we are suggesting is that, in 1901, there is no riskier province on the entire map than Belgium -- probabaly not even Trieste or the Black Sea. Belgium is a lot hotter than it looks! If you grab at it, be prepared for anything to happen.