Anniversary of the start of the war
Following the assassination of the Austrian Arch Duke Franz Ferdinand by a Serbian nationalist a month earlier, on 28 July 1914 Austria had declared war on Serbia. This set into motion a chain of alliances and reciprocal declarations of war. Two sides emerged; the Central Powers, led by Austria-Hungary, Germany, and Turkey (The Ottoman Empire), and the Allied powers led by France, Russia, and Great Britain. The fact that these nations also each ruled over large global empires dragged nations all over the world into the war.
By the end of the first year of the war, the conflict had settled into a war of attrition: Little territorial advance was being made by either side and it was now a waiting game to see which side could last the longest and bear the greatest losses.
In many ways, the first year of the war had been the worst. Traditional strategies of warfare had come up against horrific new weapons technology (including chlorine gas, used for the first time in April 1915). This had resulted in even more death and injury than had been expected. By the summer of 1915, the military leaders on all sides were taking a more cautious approach. The war was unlikely to end soon.