This is why Cossacks are Russia's legendary fighting force
For centuries, friends and foes of Russia marveled at the fierce fighting skills of their Cossack Warriors. The Cossacks fought to defend the Russian Czar against any manner of enemies – from Ottoman Turks to Napoleon's Grande Armée, through a World War, and even against the Bolshevik Red Army.
The last one had admittedly mixed results.
They expanded the borders of the Russian Empire, conquering Siberia and the Caucasus regions for the Czar, all the way to the Bering Sea – capturing one-sixth of the Earth's land area. Their martial prowess was unmatched in the region for a long time and they refused to be tied to any master. Even the name "Cossack" in the Turkic languages of the time and area meant "free," "adventurer," or "wanderer."
Cossack loyalty to the Russian Czar was earned over centuries of fighting to maintain that independence. Many Czars were faced with Cossack uprisings and were forced to deal with them in their own ways, from putting down the rebellion or forcibly moving the population to another area of the Russian Empire.
From a young age, Cossacks raised their children to be elite warriors and ethnically Cossack in every way possible. Training could begin in infancy and only ever stop in an actual pitched battle.
Keep your head down!
Eventually, the Russian nobility came to accept the Cossacks, endowing them with certain rights and privileges in the Empire for their continued service in defending Russia's borders. And they earned those rights, too. After his Grand Armée was forced out of Russia, the Cossacks harassed them all the way back to France. It was the Cossacks who captured Paris and unseated the French Emperor.
When the Empire fell during World War I and the Czar abdicated, the Cossacks were divided between the Red and White factions of the Russian Civil War. They fought primarily for the White (anti-Communist) Russians, which earned them persecution when the Bolsheviks won the war and founded the Soviet Union.
The persecution got so bad, many Cossacks fought for Nazi Germany during WWII.