Belgium's unexpected resistance delayed the Germany army and it prevented Germany from meeting their 6 weeks deadline.
Germany is never going through Belgium!
The Schlieffen Plan thought Russia would take 6 weeks to be fully armed. So Germany planned to defeat France within the 6 weeks. However, Germany's plan was ruined as Russia got armed shorter than Germany had anticipated.
The Schlieffen plan was made by Alfred von Schlieffen in 1905 and he retired in 1906. He was dead in 1914. So they couldn't make an updated version of the plan for the real war. Germany's plan was out of date.
Von Moltke reduced the troops in the east and changed the route to go through Belgium not Holland
We , the German army, will reduce the army in the east. Also we will go through Belgium instead of Holland.
Germany thought Britain would not join the war immediately which will give Germany time to defeat France. However Britain arrived in France/Belgium and slowed Germany's advance, delaying their 6 weeks plan.
We will successfully help our allies, Britain and Belgium against those evil Germans.