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Bonito
The critically acclaimed first season is Poland from 960 to 992 just when Mieszko I became the first historical prince of Poland (before that the facts are mixed with legends). In 966 the Baptism of Poland took place which made the country count internationally since it was recognized by the papacy.
The second season with a changed formula is Poland from 1333 to 1370. In 1333 Casimir III the Great was crowned. A king often considered the greatest. There's a famous saying that he upgraded Poland from a wooden fortress to a stone one. He also funded Jagiellonian University. He died in 1370.
The amazing third season with a guest star is Poland from 1371 to 1635. That's a large span of time, but the gist of it is that Poland and Lithuania created the Polish–Lithuanian Commonwealth. The Union of Krewo, the Union of Horodło and the Union of Lublin being the most important events in uniting them. Many important events took place during that time, like Poland and Lithuania totally wrecking Teutonic Knights in the Battle of Grunwald in 1410, or Prussian Homage in 1525 that finished many, many wars between Poland and Teutonic Order (the latter becoming Poland's fief).
Series cancelled, assets bought by rivaling studios refers to the First Partition of Poland in 1772. The Russian Empire, the Kingdom of Prussia and the Austria-Hungary partitioned Poland. Poland ceased to be a sovereign country for 123 years. In case you're wondering, there also was the Second Partition of Poland in 1793 and the Third Partition of Poland in 1795.
The failed fan-made reboot refers to 1807 when Napoleon created The Duchy of Warsaw. It was said to be free, but de facto it was dependent on French Empire. The Duchy ceased to exist in 1815 when Napoleon was defeated.
Series back due to popular demand, explosive season finale, almost cancelled refers to Poland from 1922 to 1939. In fact Poland regained independence in 1918, but it was 1922 that was formative for the borders. Also in 1922 the first president of Poland - Gabriel Narutowicz - was chosen. He was shot dead the same year by Eligiusz Niewiadomski, though. I think "almost cancelled" refers to the Battle of Warsaw that took place in 1920. Basically, Poland miraculously destroyed the Soviet Russia right before they entered the capital. It stopped Soviet March to the West and basically saved the entire Europe from the Reds.
Ongoing season, back to the roots with mixed reviews refers to Poland from 1945 to the present. After World War Two Poland lost many lands in the East, but gained some in the West. The East border for example was set by Stalin who decided the Curzon Line will be the border.
Cheers!