Queen of Poland and Grand Duchess of Lithuania
John Sobieski was elected King of Poland and Grand Duke of Lithuania in 1674, not without the influence of his wife. As the Queen of Poland and Grand Duchess of Lithuania, Marie Casimire quickly became unpopular, as she supported the proposed Polish–French alliance, while simultaneously striving to gain privileges for her family from the French King Louis XIV, whom she greatly admired. Like Marie Louise Gonzaga, Marie Casimire was a strong supporter of an absolute monarchy, for which she was reviled by certain spheres of the Szlachta. The nobility were scandalised at the queen's political meddling, believing that foreign women shouldn't interfere with politics. Marie Casimire also opposed the Commonwealth's policy of religious toleration and supported the Edict of Fontainebleau of 1685.
Her influence is exemplified in an incident in 1678, when the Chancellor of Nowogrod, Mikołaj Władysław Przeździecki, came to the king as the leader of an unofficial delegation to present him with evidence that the Pac family were his enemies. According to Mikolaj Wladyslaw Prze Dziecki, "The Queen knocked on the doors until the King asked for them to be opened and then took the monarch by the hand, leading him to the side, with great force, speaking in French", which the Chancellor viewed with disgust.
In the 1670s, the king of France wished to close a treaty between France, Sweden, the Polish-Lithuanian Commonwealth and the Ottoman Empire. The French ambassador recommended working with Marie Casimire since "only the Queen Consort of Poland can convince the King, her husband, to close a treaty".
Queen Marie Casimire gave her support to this issue, and a secret treaty was signed in Jaworowo on 11 June 1675, and completed by a treaty between Poland and Sweden in 1677.
Her Pro-French policies were, however, discontinued in 1678, when France had not fulfilled the terms promised her for her services, such as making her father a Peer of France, combined with the fact that Marie Casimire became more concerned with the threat of the Turks.
Queen Marie Casimire sent her lady-in-waiting and confidante Malgorzata Korowska as her political agent to the Emperor Leopold I in 1679 to find out his view concerning an alliance between the emperor and Poland against Turkey. On 1 April 1683, Poland signed the military alliance with the emperor, which she had supported.
She also played an important role during the war against Turkey, when she was credited with having convinced her spouse to support the emperor when Vienna was attacked in 1683. The French ambassador Michelle de Mongrillon described her influence when he commented:
"She could move her husband first of all, then she moved the huge, lethargic corps of the Commonwealth that is so difficult to set in motion".
Poland and Lithuania joined the Holy League in 1684, an act which had been promoted by Marie Casimire.
When King John Sobieski was absent in Wallachia and Moldova in 1686 and 1691 and at the Battle of Kamenets, Marie Casimire is acknowledged to have ruled in effect as regent in his absence.
As such, she had the authority to negotiate the military and trade treaty with France in September 1692, which restored the relations between Paris, Copenhagen, Stockholm and Warsaw, known as the "Crown of the North Alliance". She worked with the French ambassador Robert Le Roux d'Esenval and his successor Melchior de Polignac to settle unsettled affairs within the Crown of the North Alliance as well as the peace treaty with Turkey.
In 1695, she successfully solved the conflict between the King's supporter, the Bishop of Vilnius, Konstanty Brzostowski, and Kazimierz Jan Sapieha, as she wished to win the Sapieha party to support her son's candidacy as king and grand duke.
Queen Marie Casimire herself commented concerning the great political influence she had during the reign of her spouse in a letter in retrospect:
"I bear the burden of all the matters at hand, as my late husband loved me more than I deserved. Therefore, he did anything that pleased me and that I allowed for, as he considered me smarter than I am".
