Belgium has been the battleground of Europe since as far back as the Roman Empire as it had no natural protection from its big neighboring countries.
On top of that the Belgian territory is almost completely void of natural obstacles. The rivers facilitate the transport of troops and there never was a lack of roads. Ask the millions of Dutch traveling south every summer through Belgium in their cars, lugging the traditional Dutch caravan and slowing down anyone who wants to go faster than a 75 year old granny with a stroller.