WW1 was such a huge canvas that the battles and loss of life on the Western front dominate most thinking of the conflict.
It is a while back since I studied it but I believe it was Austro-Hungaria that wanted to suppress the Serbs, they needed a guarantee that Germany would stop Russian interference. The Kaiser more or less turned a blind eye. The Russians began to mobilise which essentially was a mass troop movement from north to south.
Once fully mobilised the Russians could have turned west and using mass swept through Germany. The Schlieffen Plan required that Germany defeat France and then attack Russia thus avoiding a simultaneous war on two fronts.
The main premise was that Russia could not be allowed to mobilise with France as a potential aggressor in the west, the plan's main flaw was that it was an offensive plan rather than defensive.
Another factor was that France had just increased its conscription for 2 years to 3.
Equally the British had started in 1911 to increase its Army through recruitment of a Territorial Force. Remember also the naval arms race with Britain building battleships faster than Germany.
The world in the early part of the 20th Century was not the peaceful state we are led to believe.