The union of Lublin unified the two nations of Poland and Lithuania into one nation under the Jageillonian Dynasty

The Holy League was set up by nearly all of the nations in Europe that neighbored the Ottoman Empire as a joint military force with the purpose of warding off attack by the Ottomans.

The Union of Kreva saw the marriage of the Grand Duke of Lithuania to the daughter of the recently passed Polish king. Jadwiga had no brothers so she was crowned King of Poland, set to rule until she married. Her mother then arranged a marriage with the leader of Lithuania, the Grand Duke Jogalia, thus joining the royal lines of Poland and Lithuania.

Of the Dozen members of the Holy League, only the Polish-Lithuanian Commonwealth sent an army to relive the siege of Vienna. The other nations were reluctant to become involved for fear of provoking an invasion of their own nations. 

Prussia was an electorate of the Holy Roman Empire, which meant that the leader of Prussia had a vote when it came time to elect a new leader to be the Holy Roman Emperor. The Holy Roman Empire was a loose confederation of primarily Germanic speaking nations and principalities.

The Congress of Vienna was held in the wake of the Napoleonic Wars. The nations also met with the purpose of addressing several other pressing issues Europe was faced with. The meeting began in September of 1814 and went on for nine months until the issues were settled in June of 1815.

The Congress of Vienna was convened with the purpose of reshaping the map of Europe. The participating nations wanted to reorganize Europe in order to bring back the balance of power that had existed between the nations a century earlier so as to avoid any large scale wars like the ones they had just seen during the Napoleonic Wars.

The Congress of Vienna was meant to undo the effects of the French Revolution on Europe and put royal families back in control of the nations of Europe. It also sought to give large territorial gains to the victorious powers of Continental Europe, mainly Austria, Prussia, and Russia. The idea was that if these nations were powerful enough they could keep any future threat from France in check. They also wanted to replace the Holy Roman Empire with a newer German Confederation since the Holy Roman Empire was dissolving.

While all of these men were present at the negotiations in Vienna, the chairman would be the leader of the Austrian Delegation. As Vienna was the capital of Austria, it was only logical to have Klemens Wenzel von Metternich, the Austrian Foreign Minister chair the negotiations and serve as intermediary between different delegations.

Spain was not considered one of the Great Powers. While they were signatories of the Congress of Vienna, they were not invited to take part in large parts of the negotiations. Spain's power had waned and Spain was not considered a militarily powerful nation, as it had been two centuries earlier.