Question

Why did Russia send troops to the border of Austria-Hungary in 1914? Austria-Hungary had invaded Serbia, and Russia was in an alliance with Serbia. Austria-Hungary was threatening to invade Russia. Austria-Hungary was sending its people to live in Russia. Austria-Hungary needed help resisting a German invasion.

Answer 1

Answer;
Austria-Hungary had invaded Serbia, and Russia was in a alliance with Serbia.

Russia thus sent troops to the border of Austria-Hungary in 1914 to continue the mobilization of Europe.
Explanation;
In July 28, 1914- the Austria-Hungarian empire declares war on Serbia, on July 29; Britain calls for international mediation to resolve the worsening crisis. Russia urges German restraint, but Russia begins partial troop mobilization as a precaution. Germans warns Russia and begins to mobilize themselves. 
In July 29, 1914 Austrian warships bombard Belgrade, the capital of Serbia.
Reacting to this attack on Serbia, Russia begins full mobilization of troops.  

Answer 2

Austria-Hungary had invaded Serbia and Russia was in an alliance with Serbia.
On July 28, 1914 Austria declared war on Serbia and on July 30, Russia mobilized troops on the border.