Germany quickly defeated France in the Franco-Prussian war in 1870.
In 1914 Germany included Alsace-Lorraine and had an excellent railroad system that allowed for fast mobilization of its powerful army.
Belgium was a neutral country that presented no threat to Germany.

The small Belgian army fought extremely well, providing France with invaluable time to mobilize and Britain with invaluable time to send troops to France. Even after Germany occupied most of Belgium, snipers killed many officers and Belgium provided few benefits to Germany. In contrast, Holland was not invaded and became an important supplier for Germany, smuggling goods past the British blockade.

Had Germany simply concentrated its troops along the French border, before declaring war and invaded only France the day it declared war, so that mobilization were limited, the powerful German army may have broken through to Paris before the British troops arrived in force and the French army gathered in strength.

They did win WW1.
British commanders were completely incompetent.

snipers killed many officers:in the arithmetic of Sam many means 1 or two .
Whatever :THERE WERE NO SNIPERS IN BELGIUM AND NO OFFICERS WERE KILLED .
No Belgian citizen was that foolish to shoot on a German .
About the OP :as expected,a lot of nonsens :the only chance for the Germans to defeat France,was in a short campaign(before the Russians were ready),and this only was possible,if Belgium was invaded (the Schlieffen plan) .
No Schlieffen plan would mean losing the war in 1914 .

The Schlieffen plan was extremely stupid and was modified by Moltke. Schlieffen specified invading even the Netherlands! creating yet another enemy absorbing even more German troops in otherwise neutral territory, instead of concentrating on the enemy.
Moltke modified considerably the Schlieffen plan as it was obsolete even in 1906.
One of the greatest problems in Moltke's offensive was the depleted right flank, thanks to Belgium's brave defense.
How can creating an additional enemy with several divisions and having to occupy a lot of additional territory expedite the invasion of France? Why was it not necessary in 1870?

Britain declared war on August 4, because it had not received a satisfactory response to its ultimatum requiring that Belgium be kept neutral.

It is unfortunate that a Belgian would not know about the Belgian franc-tireurs and the inhuman German reprisals in WW I, see:
http://www.firstworldwar.com/atoz/franctireur.htm

Again, your favorite word is correctly spelled nonsense in english

Before giving a source to defend your arguments,it would be better to read it :there was NO ONE exemple of franc-tireurs in Belgium . This was only German imagination .
There were no franc-tireurs in Dinant,no franc-tireurs in Louvain (only Germans shooting at each other)
And that the Schlieffenplan was stupid,of course,Sam knows better,Schlieffen was an imbecile (as the British generals)

Horne and Kramer(German atrocities in 1914:a history of denial) have given as explanation the collective fear of the People's War:the German soldiers of 1914 did not know what a war was,they knew only the (as usual )exagerated stories of the veterans about the war of 1870
"the source of the collective fantasy of the People's War and of the harsh reprisals with which the German Army(up to its highest level)responded are to be found in the memory of the Franco-Prussian war of 1870-1871,when the German armies faced irregular Republican soldiers (or Franc-Tireurs)and in the way in which the spectre of civilian involvement in warfare conjured up the worst fears of democratic and revolutionary disorder for a conservative officer corps."
"inexperience leading to lack of discipline amongst German soldiers,drunkenness,"friendly fire" incidents arising from panic,frequent collision with Belgian and French rearguards leading to confusion,rage at the stubborn and at first successful defence of Liège during the battle of lige,when German's invasion first failed,rage at Belgian resistance at all,not seen as a people entitled to defend themselves,prevailing almost hatred of the Roman Catholic clergy in Belgium and in France,ambiguous or inadequate German field service regulations regarding civilians,failure of German logistics later leading to uncontrollled looting,etc "
BUT :NO Belgian franc-tireurs .I dare you to cite one exemple of Belgian franc-tireurs shooting on Germans .
The Belgian population knew instinctively what would happen during an invasion (since 1000 years,Belgium was the battle field of Europe):inexperienced and panicked soldiers would rape,loot,shoot civilians.
On a much lesser scale,the same happened in may 1940,when there were a lot of trigger-happy Belgian,French and British soldiers,believing all the nonsense about the fifth column

Schlieffen was a 19th century man planning a 20th century invasion. His plan did not even mention logistics. But of course being comissioned by the incompetent Kaiser (who ruined all the alliances and achievements of Bismarck) he couldn't help to be brilliant.

It is interesting that both in Belgium in 1914 and in Poland in 1939 the German offensives ran out of ammunition. Guderian mentions this problem in Belgium in Achtung, Panzer! He also analyzes the waste of excellent cavalry and infantry because of outdated tactics that caused much greater German losses than Belgian losses in several battles and the fact that it was rather silly of both sides to try to defend all the conquered territory with heavy losses, instead of moving the lines to more defensible positions (Hitler would make the same mistake in the USSR when he ordered several times to defend the positions to the last men, rather than allowing a fluid defense).
Saving scarce ammunition to attack France, instead of otherwise neutral territory is yet another reason for not invading Belgium in 1914.