I have a basic idea about the Schieffen plan and I know that Germany attacked Belgium in the First world war. But I am slightly confused about this topic, as despite the fact that the schieffen plan did involve taking the French by Surprise, a proposal with Belgium to let the German troops pass would’ve been a lot faster and would’ve meant less blood-shed and gore. This raises the chance that Germany, may instead have been trying a country expansion or a conquest just like they did with Alsace and Lorraine. There is one other chance, which is whether Germany attacked Belgium because they had a treaty with Britain where if they were attacked Britain would step in (Britain were in the triple entente). So, I was just wondering which of this is the most likely reason as to why Germany chose to attack neutral Belgium?
The Germans in fact made such a request. Of course the word "request" in this context should really have irony quotes, considering the disparity in power between the two countries, and the fact that Germany was not going to take "no" for an answer.
So bascially the Germans attacked because attacking France through Belgian territory was The Plan, and the Belgians refused to just let them walk through unopposed.
It may seem absurd to us today, but in 1914 the German General Staff fully believed that it was possible to march through Belgium and simultaneously maintain her neutrality. The Schlieffen Plan was in fact premised on this assumption.
You rightly ask how this absurd notion arose - in Napoleon's 1805 campaign when the French marched through Ansbach-Bayreuth, then Prussian territory, en route to surrounding Mack's forces at Ulm. Napoleon steadily maintained that such march did not violate Prussian neutrality because the French troops were merely in transit. While the Prussian's blustered at this, and a year later cited it as a casus belli, none-the-less Napoleon's dramatic victory at Austerlitz resulted in them, temporarily at least, accepting Napoleon's argument. (Note the line of march to Ingolstadt for the corps of Marmont, Bernadotte, and Deroi.)
Having been the victim of this ruse in one war against the French, they thought it was fair game to employ it themselves, tit-for-tat, in another.
This was of course naïve in the extreme. Belgian Neutrality was explicitly guaranteed by the Treaty of London (1839) of which Prussia, as a member of the German Confederation, was a signatory. The intervening eleven decades had dramatically altered the political dimensions of warfare due to the emergence of Nationalism in the latter stages of the Napoleonic era. The atrocities committed by German soldiers against a Belgian populace outraged by the violation of their neutrality quickly turned worldwide popular opinion against the Germans. And the troops quickly assigned to protect supply lines through Belgium were first-line troops diverted from the Schlieffen Plan's big right wing, and were sorely missed on The Marne.
An important lesson to draw from this is the need for civilian oversight of military planners in our age. No longer can military expediency be regarded as sufficient motivation for military action. While the Prussian (and then German) General Staff was far and away the most capable in the world technically through the decades from 1840 to 1945, it's failure to accept or even recognize the need for civilian oversight post-Bismarck was repeatedly it's undoing.