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Late reign

Emperor Francis died on 18 August 1765, while he and the court were in Innsbruck celebrating the wedding of his second surviving son, Leopold. Maria Theresa was devastated. Their eldest son, Joseph, became Holy Roman Emperor. Maria Theresa abandoned all ornamentation, had her hair cut short, painted her rooms black and dressed in mourning for the rest of her life. She completely withdrew from court life, public events, and theater. Throughout her widowhood, she spent the whole of August and the eighteenth of each month alone in her chamber, which negatively affected her mental health. She described her state of mind shortly after Francis's death: "I hardly know myself now, for I have become like an animal with no true life or reasoning power."

Upon his accession to the imperial throne, Joseph ruled less land than his father had in 1740, since he had given up his rights over Tuscany to Leopold, and thus he only controlled Falkenstein and Teschen. Believing that the emperor must possess enough land to maintain his standing as emperor, Maria Theresa, who was used to being assisted in the administration of her vast realms, declared Joseph to be her new co-ruler on 17 September 1765. From then on, mother and son had frequent ideological disagreements. The 22 million florins that Joseph inherited from his father was injected into the treasury. Maria Theresa had another loss in February 1766 when Haugwitz died. She gave her son absolute control over the military following the death of Leopold Joseph von Daun.

According to Austrian historian Robert A. Kann, Maria Theresa was a monarch of above-average qualifications but intellectually inferior to Joseph and Leopold. Kann asserts that she nevertheless possessed qualities appreciated in a monarch: warm heart, practical mind, firm determination and sound perception. Most importantly, she was ready to recognise the mental superiority of some of her advisers and to give way to a superior mind while enjoying support of her ministers even if their ideas differed from her own. Joseph, however, was never able to establish rapport with the same advisers, even though their philosophy of government was closer to Joseph's than to Maria Theresa's.

The relationship between Maria Theresa and Joseph was not without warmth but was complicated and their personalities clashed. Despite his intellect, Maria Theresa's force of personality often made Joseph cower. Sometimes, she openly admired his talents and achievements, but she was also not hesitant to rebuke him. She even wrote: "We never see each other except at dinner... His temper gets worse every day... Please burn this letter... I just try to avoid public scandal." In another letter, also addressed to Joseph's companion, she complained: "He avoids me... I am the only person in his way and so I am an obstruction and a burden... Abdication alone can remedy matters." After much contemplation, she chose not to abdicate. Joseph himself often threatened to resign as co-regent and emperor, but he, too, was induced not to do so. Her threats of abdication were rarely taken seriously; Maria Theresa believed that her recovery from smallpox in 1767 was a sign that God wished her to reign until death. It was in Joseph's interest that she remained sovereign, for he often blamed her for his failures and thus avoided taking on the responsibilities of a monarch.

Joseph and Prince Kaunitz arranged the First Partition of Poland despite Maria Theresa's protestations. Her sense of justice pushed her to reject the idea of partition, which would hurt the Polish people. She even once argued, "What right have we to rob an innocent nation that it has hitherto been our boast to protect and support?" The duo argued that it was too late to abort now. Besides, Maria Theresa herself agreed with the partition when she realised that Frederick II of Prussia and Catherine II of Russia would do it with or without Austrian participation. Maria Theresa claimed and eventually took Galicia and Lodomeria; in the words of Frederick, "the more she cried, the more she took".

A few years after the partition, Russia defeated the Ottoman Empire in the Russo-Turkish War (1768–1774). Following the signing of the Treaty of Küçük Kaynarca in 1774 that concluded the war, Austria entered into negotiations with the Sublime Porte. Thus, in 1775, the Ottoman Empire ceded the northwestern part of Moldavia (subsequently known as Bukovina) to Austria. Subsequently, on 30 December 1777, Maximilian III Joseph, Elector of Bavaria died without leaving any children. As a result, his territories were coveted by ambitious men, including Joseph, who tried to swap Bavaria for the Austrian Netherlands. This alarmed Frederick II of Prussia, and thus the War of the Bavarian Succession erupted in 1778. Maria Theresa very unwillingly consented to the occupation of Bavaria, and a year later she made peace proposals to Frederick II despite Joseph's objections. Although Austria managed to gain the Innviertel area, this "Potato War" caused a setback to the financial improvement that the empress had made. The 500,000 florins in annual revenue from 100,000 inhabitants of Innviertel were not comparable to the 100,000,000 florins that were spent during the war.

It is unlikely that Maria Theresa ever completely recovered from the smallpox attack in 1767, as 18th-century writers asserted. She suffered from shortness of breath, fatigue, cough, distress, necrophobia and insomnia. She later developed edema.

Maria Theresa fell ill on 24 November 1780. Her physician, Dr. Störk, thought her condition serious, although her son Joseph was confident that she would recover in no time. By 26 November, she asked for the last rites, and on 28 November, the doctor told her that the time had come. On 29 November, she died surrounded by her remaining children. Her body is buried in the Imperial Crypt in Vienna next to her husband in a coffin she had inscribed during her lifetime.

Her longtime rival Frederick the Great, on hearing of her death, said that she had honored her throne and her sex, and although he had fought against her in three wars, he never considered her his enemy. With her death, the House of Habsburg died out and was replaced by the House of Habsburg-Lorraine. Joseph II, already co-sovereign of the Habsburg dominions, succeeded her and introduced sweeping reforms in the empire; Joseph produced nearly 700 edicts per year (or almost two per day), whereas Maria Theresa issued only about 100 edicts annually.