Francis Stefan of Lorrain succeeded his father as Duke of Lorraine in 1729. Maria Theresia of Austria had organized in 1732 that Francis became "Lord Lieutenant" (locumtenens) of Hungary. He was not exited about this position, but Maria wanted him closer to her. In Juni 1732 he agreed to go to Presburg.
The emperor Charles VI, Holy Roman Emperor, at the end of the War of the Polish Succession, agreed to compensate the French candidate Stanislaus Leszczynski, (father-in-law of Louis XIV of France) for the loss of his crown in 1735.
France's prime minister, Cardinal Fleury, saw the Polish struggle as a chance to strike at Austrian power in the west without seeming to be the aggressor. While he cared little for who should become King of Poland, the cause of protecting the King's father-in-law was a sympathetic one, and he hoped to use the war as a means of humbling the Austrians, and perhaps securing the long-desired Duchy of Lorraine from its duke, Francis Stephen, who was expected to marry Emperor Charles's daughter Maria Theresa, which would bring Austrian power dangerously close to the French border.
A preliminary peace was concluded in October 1735 and ratified in the Treaty of Vienna in November 1738. Augustus was confirmed as king of Poland, Stanisław was compensated with Lorraine (which would pass on his death, through his daughter, to the French), while the former Duke of Lorraine, Francis Stephen, was made heir to the Grand Duchy of Tuscany, which he inherited in 1737.
Although fighting stopped after the preliminary peace in 1735, the final peace settlement had to wait until the death of the last Medici Grand Duke of Tuscany, Gian Gastone in 1737, to allow the territorial exchanges provided for by the peace settlement to go into effect.
In March 1736 the Emperor persuaded Francis, his future son-in-law secretly (!) to exchange Lorraine for the Grand Duchy of Tuscany. France had demanded that Maria Theresa's fiancé surrender his ancestral Duchy of Lorraine to accommodate the deposed King of Poland. The Emperor considered other possibilities (such as marrying her to the future Charles III of Spain) before announcing the engagement of the couple. If something would go wrong, Francis would become governor of the Austrian Netherlands.
Elisabeth of Parma had also wanted the Grand Duchy of Tuscany for her son Charles III of Spain; the Grand Duke of Tuscany, Gian Gastone de' Medici was childless and was related to Elisabeth via her great grandmother Margherita de' Medici. As a result Elisabeth son's could claim by right of being a descendant of Margherita.
On January 31, 1736 Francis had agreed to marry Maria Theresia. He hesitad three times (and laid down the feather before signing). Especially his mother Élisabeth Charlotte d'Orléans and his brother Prince Charles Alexander of Lorraine were against the loss of Lorraine. On February 1, Maria Theresia send Francis a letter: she would withdraw from her future reign, when a male successor for her father would appear.
They married on February 12 in the church in Vienna (Hofkirche). The wedding was held on February 14, 1736. The (secret) treaty between the Emperor and Francis was signed on May 4, 1736. On January 24, 1737 Francis received Tuscany from his father-in-law.Cite error: There are <ref> tags on this page without content in them (see the help page). Until then, Maria Theresa was Duchess of Lorraine.
Gian Gastone de' Medici, who died on 9 July 1737, was the second cousin of Francis. [citation needed] In June 1737 Francis went to Hungary again to fight against the Turks. In October 1738 he was back in Vienna. In December 17, 1738 the couple travelled south, accompanied by his brother Charles to visit Florence for three months. They arrived on January 20, 1739.
In 1744 Francis' brother Charles married a younger sister of Maria Theresia, Archduchess Maria Anna of Austria (1718–1744). In 1744 Charles became governor of the Austrian Netherlands, untill his death in 1780.