To race in Belgium as an American cyclist you need at least 2 things besides your bike:
1. A UCI, so-called "international" license. You can get this license on the USA Cycling website for $150. If you plan to race in America and go abroad, just get a UCI license. You can race state-side with it too. I noticed earlier this year that when you order a UCI license and you already have a regular license, USA Cycling will deduct the price of the regular license. I was a bit surprised when that happened because I had been told you better order the UCI license so as to avoid paying twice. In either case, it never hurts to order a UCI license.
2. A foreign permission letter. This letter can be ordered at the same time as the UCI license and only costs an additional $5. It basically states that you are a rider in good standing and will be visiting the foreign country. It also states that you are allowed to affiliate with any team when doing so. You will need the letter to enter a race although I also noticed that not all races ask for it.
These are important documents so it is helpful to make copies and keep these in a different spot. In a pinch, a copy may do the job so keep one around.
If you are under 19 yrs of age (racing age) you also need another document. This document is called a "kalenderkaart" and you need to order it from the KBWB. You can use the web to do so, it is under "formulieren" (forms).
The kalenderkaart is used to keep track of all the races you do. The reason you need it is that racing for youths is subject to restrictions. I will discuss this later.
The kalenderkaart costs EUR12 and you won't be able to race without it. Although it is easy to order on the web, getting the money wired can be a problem. Be especially aware of outrageous bank fees or delays.