In October of 1915, a massive joint offensive of the Austro-Hungarian, Bulgarian and German forces, commanded by the German General von Mackensen, overwhelmed the Serbian army and completed the conquest of the Balkans. After more than a year of resistance, the Serbs had recover to the Greek island of Corfu, under the protection of the Allied forces.
The ultimatum to Serbia triggered the system of alliances that, years before, had been judged the best deterrent against the outbreak of war. Thus, the last hope of isolating the conflict in the Balkans failed: the July crisis had begun. Europe was just one step away from disaster.