In the Pax Britannica, between 1851-1914, before the "War to End All Wars," there was relative peace. However, tensions were building up in the Balkans, and Russia was temporarily reeling from defeat in the Russo-Japanese war of 1905.
In the Balkans, there was already unrest for quite a while. In 1908, Austia-Hungary annexed Bosnia and Herzegavina. In addition to this, they already had other territories in the Balkans, which caused unrest. Up to the outbreak of WW1, nations were itching for an excuse to have a war. And they got it.