OK, if people are going to talk about this, can we at least talk about generally accepted history?
History
Generally, Poland's official 'start' as a kingdom was in 966. As usual, these 'official' dates are a little arbitrary, but it is viewed as when Miesko I, a leader of a Slavic group the 'Poles', accepted Christianity.
Poland's leader, if it were included in Civ, should probably be Boleslav Chowbry (the Brave), Miesko's son. He conquered/united the areas we view pretty much as Poland today. At this point, Poland was a powerhouse in Eastern Europe.
Poland suffered a bad defeat to the Empire in 1031, but reformed under another great King, Casimir. Casimir's son (Boleslav the Daring) further strengthened Poland, and they were again a great power. Pope Gregory agreed to call Poland a Kingdom, and it became the Kingdom of Poland in 1076.
The 12th century started to see Poland lose its power, for the usual reason; internal fragmentation and fighting by the ruling family. Indeed, later, Poland would lose its status as a world power because of poor political control, but I get ahead of myself .....
Even if Poland had been united, it is hard to say if they could have staved off the disaster known as the Mongols, who crushed the Poles in 1241. The Mongol invasion had lasting effects -- the depopulation of Poland was so great that many Germans moved into Poland in the 13th Century.
Also in the 13th Century, one of the Polish Kingdoms invited the Teutonic Knights to help fight the Prussians, a pagan group on Baltic Coast. Unfortunately for the Poles, the cure was worse than the problem, and the Knights occupied a large part of Prussia. Then, war with Poland became a constant problem.
However, in the 14th Century Poland was again strong enough to expand east, as the Russian kingdoms were not really strong enough to resist.
The 14th Century was a good one for Poland. The key event probably was the unification of Poland under Ladislas I, and his son, Casimir the Great, another good choice as Poland's overall leader. Of course, all fo Europe was recovering fromthe Black Death, but Poland grew powerful.
1385 starts the Golden Age for Poland, as Poland formed a union with Lithuania (sharing kings. Formal union was 200 years away, however, after the Treaty of Lublin). This was for mutual support against the Teutonic Knights and the growing Russian (Moscovy) power in the East. However, something real bad happened also then ....
Casimir died without an heir, and Poland set up a Constitutional Monarchy. This may seem like a 'good thing', but the problem was that the Polish constitution gave too much power to the nobles relative to the king.
Before I go on, let me say that Poland's power came from its elite cavalry. The would march from the Vistula to the Carpathians, controlling large tracks of territory. However, this army form required unity with the nobles, and Polish cavalry was as good as anything Europe produced.
At his point, Poland/Lithuania was the premier state in Eastern Europe. Indeed, one could argue they were the greatest power in Europe, perhaps maybe not quite as powerful as Venice and a few others, but certainly up there.
In 1410, the Poles decisively defeated the Teutonic Knights, who ceded roughly half of Prussia to Poland, and the rest of Prussia became a client state.
From 1410 to 1585, Poland ruled virtually from the Vistula to modern day Ukraine, again with the cavalry being their key arm.
But the mid-16th century they had a problem. Their wonderful cavalry ran into their traditional tormenters, the Mongols. Even though the Crimean Tatars weren’t the power of Genghis Khan, they were a constant thorn to Poland and fought on and off for the next 200 years.
In 1573, they went to 'elect' a king for life, with all nobles appearing at Warsaw getting a vote. They chose Henry of Valois, the brother of Charles IX of France, a natural Catholic ally. However, Henry took the French crown instead a few months later when Charles died. At the time, most people felt that Poland was a richer and stronger power than France. Once again, the problems of a strong king emerged.
Poland also started to suffer Cossack Rebellions. The Cossacks went over to Russia as protection, and Poland also fought a long series of wars with its rising rival in the East.
Poland's power was broken by a series of events called 'The Deluge', usually viewed as 1655 1660 (but, as usual, hard dates are a bit unscientific). Poland faced a Cossack rebellion (which, unfortunately, was the beginning of the huge Cossack pogroms against the Jews that went on unrelenting for the next 250 years), and simultaneous invasion by Sweden and Russia.
But Poland was still a power, and, slowly, they defeated the Cossacks, the Russians, and the Swedes! They also defeated Prussia, but Prussia was given independence.
Once again, Poland found internal dissention during the Deluge a key problem. Some Poles allied with the various invaders.
Nonetheless, Poland was not a great power or perhaps even a secondary power. By now, Spain, France, England, Austria, Russia, and Sweden were far more powerful.
The final blow to Polish power cam from the rise of Prussia, as the former 'Prussia' became part of the rising power of Brandenberg. The new 'Prussia' became the central state in Germany, and between 1772 and 1795 Poland was split between Austria, Prussia, and mostly Russia.
Why didn't they fight? Poland's constitution allowed the Nobles too much freedom, and Poland couldn't go to war even under these circumstances without support from the nobles. Of course, the Russians controlled many of these nobles, and Poland gave up without a fight.
Poland, of course, was freed after WWI, since its tormentors (Prussia/Germany, Austria, and Russia at first) were not in position to fight it out.
But Poland would have a few more days in the sun ….
After the end of the Russian Civil war and the establishment of the Soviet Union, the new regime wanted to end the independence of the Poles that was imposed on them when they didn’t have the strength to fight it. But now, the new Soviets were looking at the new Poles and the defeated Germany and the fervor of spreading Communism after defeating the various forces trying to defeat them during the civil war became a key goal. One of the most important, underrated wars, the Polish Soviet war, was about to begin.
In 1918 and 1919, Poland defeated the Ukraine, as Poland tried to regain some of its historic Eastern territories. In February 1919, however, Poland was at war with Russia. Its pretty hard to state definitively what started it, as Poland tried to move East and Lenin tried to regain the former Russian lands. Conflict was inevitable. It was a very complex war (the Poles preferred the Bolsheviks to the Whites, for instance). Originally, Poland moved far into the east, getting into the Ukraine.
The key to the Soviet counter-attack was the defeat of Denikan (whites). The Soviet counterattack drove them back, all the way to the Vistula. Then, the ‘miracle of the Vistula’ a gap developed between the two attacking Soviet armies, and Poland counter-attacked again, and drove the Soviets back.
At the time of the armistice, Poland had the upper hand, but only slightly. Both sides claimed victory. However, the war is considered pivotal. Had the Soviet power, after defeating the whites, crushed Poland, they may have moved against the defeated Germany.
In WWII, Poland fought well. Poland was doomed by the fall of Czechoslovakia in WWII, leaving them pretty much undefendable. Poland inflicted a large number of casualties nonetheless on the Germans. And it was the Poles who broke the Enigma code, and gave the solution to the British. We can speculate how WWII would have turned out without the Polish mathematicians who broke the enigma secret.
Analysis
Poland was a great power for a long period. From the late 10th century until the 17th , Poland was a major power in Eastern Europe. They were a lesser power in the 12th and 13th century, and by the mid 17th they weren’t a great power probably. Clearly, when they merged with Lithuania they were a world power, and they had this status maybe for 200 years.
Why did Poland lose its power? Here, of course, we now go from recitation of history to opinion. Here is my opinion. The main problem seems to be that Poland’s power was based effectively on what we would think of a feudal basis. They had a powerful cavalry and a strong nobility that limited the power of its kings. With strong kings, they were quite powerful.
But the nearby states emerged from Feudalism, and developed strong middle classes and economies. They developed strong economies, had large rises in population and wealth, and technology developed. Spain in the 15th Century and Sweden in the 16th century revolutionized warfare, and developments were made in other countries. So, Poland did not so much as ‘get weaker’ at its core but it missed the powerful gains made by its neighbors. Furthermore, as a power in Eastern Europe, it suffered from issues that would impact the Habsburg power, revolts from ethnic groups (the Cossacks especially) and had to deal with a minority rule.
So, Austria, Russia, and Prussia-Brandenburg and Sweden emerged as powers, and Spain, France and England became even greater powers.
The final problem was that its governmental system couldn’t allow and accommodate the changes that Poland would have needed to make to grow as the other powers did. So, beset by Mongols, Cossacks, and powerful neighbors, with an internal government that was fractious, they became a minor power.
Do they belong is Civ? Maybe. People claiming they weren’t a power probably aren’t thinking about the medieval period where they were a powerhouse, and were a power as long as many of the Civ’s that are included. I’ve written about politics and military power (because it is my expertise), not about other fields like science and arts, which I leave to more learned people in these fields. My own view is that they are a good choice but not a ‘must include’ power like Rome.
Best wishes,
Breunor