It just surprises me that Russian landed nobility would be willing to arm and train hundreds of thousands of their thralls.
This picture of the serfdom and the Russian army is not quite accurate, but not surprising since the nature of Russian military service is often the subject of a distorted picture of the Russian state. Russia was atypical for an ancien regime state in that it had a somewhat coherent system of conscription and military service, albeit one that grew around Russia's extensive systems of unfree labor.
The Russian state began to require a quota of its population for military service starting in the Petrine era. Peter the Great issued a decree in 1699 requiring a fixed quota of military recruits from his nobles and other social estates with exemptions for certain estates like the clergy or newly settled regions that needed their serfs. Non-Russian populations like the Cossacks had their own special compacts of military service with the Romanovs, which was in keeping with the imperial relationships with servitor estates. The burden of military service largely fell onto the serfs as nobles offered up their serfs to meet these quotas. Although the proportions for quota system varied throughout the eighteenth century, falling in peace and being raised during war, the system evolved into a reasonably effective system given the standards of the time and the administrative poverty of the tsarist state. The conscription apparatus evolved into a civil-military/state interrelationship between local governors and provincial military and commanders and the local serf commune, the mir.
It was the mir that usually made the decision for which unlucky souls would be chosen for twenty-five years service to the tsar. Soem rich serfs could hire out a replacement, Btu this was often a difficult process, especially if the replacement did not meet the physical requirements. The selection process was often very complex and involved the internal politics of the commune. Because a recruit would likely never would return to the village, taking a healthy young male could destroy a serf household, so the patriarchs of the mir tried to select the men which the community could bear to lose. This sometimes created tensions with state authorities as these recruits sometimes did not meet the physical standards needed for military service.
Once selected for the levy, the serf-recruit usually had a funeral for him in his community. While this might seem to be a sign of passive resistance to the tsar's blood tax, it is worth emphasizing that as a soldier of the tsar, a recruit left the estate to which he was born, a relative rarity in tsarist Russia. Firstly, given the size of the empire, the limited transportation infrastructure, and the tactic of stationing the formations away from where they were recruited, it was highly unlikely a soldier could ever return home to his native village. Secondly, entering into the army meant leaving one's native estate (soslovie) and entering into a new soslovie class, often as raznochintsy (a kind of catch-all estate, not quite noble, but above serfdom). The soslovie table of ranks in the eighteenth century was not as hardened and defined as it would become in the nineteenth, but military men, even lowly infantry men, were in a separate social estate set up for military service. This estate status was inheritable and was in evidence in many of the garrison towns that serviced the Russian state. It's important to remember that garrison duty was much more the typical life of a soldier than one of constant battles and garrison duty became one of the focal points of a soldier's new life. The children of soldier's wives could enter into garrison schools, first founded in 1721, that fast tracked young boys for military service. Similarly, off duty soldiers were legally allowed to practice a trade and if they were allowed to muster out, often became tradesmen or other townsmen.
The Russian state also allowed for volunteers to the ranks both from the Great Russian population, and also from foreign prisoners of war. In the latter case, this was in line with the Romanov's policy of "attract and hold" for foreign expertise. The state also began to impress vagabonds or those individuals who did not possess an internal passport to fill up the quotas, but many within the army were leery about letting in a criminal or unreliable element within the institution.
Overall, the Russian conscription system worked reasonably well given both the size of the Russian state and the extreme length of service. There discrepancy between the called for quotas and actual recruits, but this is not surprising given the rudimentary nature of the tsarist state apparatus which often had to delegate to local elites rather than a trained bureaucracy. Potential recruits might potentially self-mutulate themselves to avoid service, removing the front teeth was common because it would been they could not tear their muskets' cartridge, but such evasions were within manageable levels. Desertion was a problem and the tsarist state sought to proactively deal with this by deliberately stationing recruits well away from their birthplace, so they would have no real safe refuge. Peasant revolts, most notably the Pugachev Rebellion, were a persistent concern for the Romanovs in the eighteenth century, but the conscription system was only one of several grievances that the serfs had against the Russian state and their nobles such as the nobles' monopoly on the production and sale of alcohol.
One of the most salient aspects of the eighteenth and early nineteenth-century Russian army was that it was a conscript system that worked within the existing social estate apparatus. Even though serfs were at the bottom pf the soslovie pecking order, they serfs still had collective rights as a social estate in the Russian empire and both the state and the nobles had to meet certain legal obligations to the serfs (this is why comparing serfdom to chattel slavery of the Atlantic is often a futile exercise because both are very different forms of unfree labor). The levy process did not fundamentally alter the mir patriarchs' hold over their community, and in some cases, could cement it by promoting some families over others or getting rid of malcontents. The long-term nature of military service also was atypical of the era, but was a logical response to the military needs of the Romanovs, who needed both active military troops and garrison soldiers that could act as military colonists for the expanding empire. This estate system imparted to the Russian army a much greater and cohesive institutional character than many of its contemporaries and the long-term service helped foster the evolution of the Russian military estate into a distinct class in Russian society.
Sources
Duffy, Christopher. Russia's Military Way to the West: Origins and Nature of Russian Military Power, 1700-1800. London: Routledge & Kegan Paul, 1981.
Keep, John L. H. Soldiers of the Tsar: Army and Society in Russia, 1462-1874. Oxford: Clarendon Press, 1985.
Stoker, Donald J., Harold D. Blanton, and Frederick C. Schneid. Conscription in the Napoleonic Era: A Revolution in Military Affairs? London: Routledge, 2009.
Wirtschafter, Elise Kimerling.From Serf to Russian Soldier. Princeton, N.J.: Princeton University Press, 1990.