Short answers, pulled from the story.
Peter the Great issued a decree in 1699 that created the basis for recruiting soldiers and marked the beginning of a transition from older Streltsy forces to new Western-style regiments. The standing army existed before the end of the Great Northern War in 1721.
In 1805, the Russian Army had many characteristics of Ancien Régime organization with no permanent formation above the regimental level and senior officers recruited from aristocratic circles. Both Russians and Austrians met a decisive military defeat at the hands of Napoleon during the Battle of Austerlitz on the 2nd of December 1805.
Following Russia's defeat in the Crimean War during the reign of Emperor Alexander II, Count Dmitry Milyutin instituted a series of military reforms based in the emancipation of the serfs in 1861. These reforms included reorganizing the Ministry of War and creating new technical and support organizations alongside changes to finances.
On the 1st of January 1874, the emperor approved a conscription statute that made military service compulsory for all 21-year-old males. The term was reduced for land army to six years plus nine years in reserve and created a large pool of experienced military reservists ready to mobilize in case of war.
During World War I, Cossacks made up two-thirds of Russian army cavalry contributing over 500,000 men for the war effort with 200,000 being Don Cossacks. They originated as Slavs that chose to live on the steppe instead of integrate with either the tsardom or Polish Lithuanian Commonwealth and were organized into several voisko named after regions across the empire.
In total, 15 million men served from 1914 to 1917 during World War I while the peacetime standing army had been estimated at around 1,300,000 before the conflict. Officers increased from 40,000 to 80,000 during the war and an estimate in 1913 put total active and reserve troops at 5,962,396 including first and second line reservists never on active duty.