If the Belgians would've let the Germans pass, World War 1 would've been over in a jiffy, making it a war not worth learning about. Saving millions of soldiers who wasted their lives in mud and blood for nothing.

It would also prevent World War 2 and the collapse of the German Bond across Europe, bringing progress, industry and wealth to East-Europe through economic endeavors.

And in case World War 2 would happen for a reason, the Belgians just have to let them pass again, making sure no infrastructure would be damaged, or people killed in the fighting. Ending the war against France even faster.