I'm really impressed with volume of this work, but I cannot understand, why Ukrainian cossacks not mentioned at all (mixed with USSR), while really saved indepe

I'm really impressed with volume of this work, but I cannot understand, why Ukrainian cossacks not mentioned at all (mixed with USSR), while really saved independent culture and independent mind. To be exact, peak of cossacks was about 1655 year (yes, unions with Poland and Lithuanian), after which happen quasi-voluntary merge with Russians, and from that time nearly constantly exists Ukrainian anti-empire resistance, mostly eliminated only in mid 1950s, with build of Kakhovka dam, which just flood cossacks.