By the end of the Napoleonic Wars Russia was the preeminent military Land Power in Europe. Their regular Army and Imperial Guard Regiments gaining a fearsome reputation for their hardiness and bravery. They also were able to learn from their mistakes after repeated losses.
After the Napoleonic wars tho there seemed to be a steep decline in their fighting prowess, tactics and quality, the Crimean war being a prime example. The Cossacks aside, their cavalry, which was excellent in the Napoleonic wars, became known as being "as bad as it could be". The Imperial Guard, their record being extemly impressive in the Napoleonic wars, is in the later years described as useless.
What the hell happened here? What lead to this decline from one of the most powerful effective fighting force to a tumbling and bumbling giant?
Reasons for this decline lie in the field of societal stagnation. After Napoleonic Wars, Russian nobility did exactly what it shouldn't have done: ignore all the modernization efforts and stall in one place with new military reforms being extremely poorly conducted. By the end of Napoleonic wars, Russian Imperial Army was indeed one of the best armies in Europe, possessing both impressive numbers and large amounts of experienced troops and officers. After the conflict ended, Russia took its place as a new "gendarme of Europe" hoping to preserve status quo (even helping Austria with Hungarian revolt). The problem here was that Russia was also seeking ways to ease up its economical burden since being primarily focused on the serf labour didn't make its economy notoriously effective. This led to Alexander I and count Arakcheev attempting to install military settlements in Russia, places where recruits could be used as cheap labour force and also be trained as soldiers. Dispersing between two notoriously different disciplines didn't end well which was coupled with rapid technological advances in 19th centuries: with lack of necessary funds and complete inability of higher leadership to enact global changes it was a matter of time before widespread military degradation would turn Russian Army into the shadow of its former self. We should also remember that during this time Russia was considered to be one of the biggest powers in Europe, a power other nations would want to challenge in the future. No wonder that combination of these factors led to explosive situation like the Crimean War.
Arakcheevshina (contemptuous monicker given to the era of military settlements) was eventually undone in 1857 after its disasterous results but the damage has already been done and economical and technological lag of Russia proved to be unbeatable for its passive nobility. It eventualy led to the collapse of Russian Empire during the revolution of 1917.
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