Answer:
Russia sent troops to the border of Austria Hungary in 1914 because Austria Hungary had declared war on Serbia, which was an ally of Russia.
Step-by-step explanation:
The First World War was a confrontation that began on July 28, 1914 and ended on November 11, 1918, when Germany accepted the conditions of the armistice.
It received the qualification of world-wide because all the great industrial and military powers were involved, divided in two alliances. On the one hand, the Triple Alliance formed by the Central Powers: the German Empire and Austria-Hungary. Italy, which had been a member of the Triple Alliance together with Germany and Austria-Hungary, did not join the Central Powers, because Austria, against the agreed terms, was the aggressor nation that unleashed the conflict. On the other hand was the Triple Entente, formed by the United Kingdom, France and the Russian Empire, which was allied with Serbia.
The trigger for the conflict occurred on June 28, 1914 in Sarajevo with the assassination of Archduke Franz Ferdinand of Austria, at the hands of Gavrilo Princip, a young Serbian nationalist. This event sparked a diplomatic crisis when Austria-Hungary gave an ultimatum to the Kingdom of Serbia and invoked the various international alliances forged over the previous decades. In a few weeks, all the great European powers were at war and the conflict spread to many other geographical areas.
On July 28, the Austro-Hungarians began hostilities with the attempted invasion of Serbia. While Russia was mobilizing to help its ally, Germany invaded Belgium, which had declared neutrality, and Luxembourg on its way to France. The violation of Belgian sovereignty led the United Kingdom to declare war on Germany. The Germans were stopped by the French a few kilometers from Paris, and a war of attrition began in which the lines of trenches hardly suffered any variation until 1917, when the United States entered the conflict.