Short answers, pulled from the story.
The Russo-Turkish War began due to deteriorating Ottoman administration in the Balkans, severe droughts and floods causing famine, and a series of uprisings starting in Herzegovina in July 1875. Russia declared war on the Ottomans on the 24th of April 1877 after Romania allowed Russian troops to pass through its territory.
Russian forces were led by several commanders including Tsarevich Alexander Alexandrovich who commanded the Eastern Detachment and Count Joseph Vladimirovich Gourko who led the Advance Detachment. The 1st Caucasus Army Corps was under Grand Duke Michael Nikolaevich while Lieutenant-General Ter-Gukasov initiated assaults into Ottoman territory from Yerevan.
Russia entered into the Treaty of San Stefano on the 3rd of March 1878 which recognized the independence of Romania Serbia and Montenegro and granted autonomy to Bulgaria. This treaty was later modified by the Congress of Berlin which split Bulgaria according to agreements among the Great Powers.
The Russian army contained 754,265 men in 1874 and grew to over 1.5 million men by the 1st of January 1878. Approximately 300,000 troops were within reach for sending into the Balkans at the beginning of the war.
During the Battle of Stara Zagora in July 1877 Suleiman Pasha's forces burned down and plundered the city subjecting its population to indiscriminate slaughter. The number of massacred Christian civilians is estimated at between 14,000 and 14,500 making it the biggest war crime in modern-era Bulgaria.