Sheridan was promoted to lieutenant general on March 4, 1869. In 1870, President Grant, at Sheridan's request, sent him to observe and report on the Franco-Prussian War. As a guest of the King of Prussia, he was present when Napoleon III surrendered to the Germans, which was gratifying to Sheridan following his experiences with the French in Mexico. He later toured most of Europe and returned to the U.S. to report to Grant that although the Prussians were "very good brave fellows [who] had gone into each battle with the determination to win, ... there is nothing to be learned here professionally." He criticized their handling of cavalry and likened their practices to the manner in which Meade had attempted to supervise him. However he referred to theirs as a "perfect military system" and had a high opinion of the officer corps. His words on the French were much more harsh; he criticized the French army for not taking numerous opportunities to halt the German advance, for advancing slow and clumsily themselves, for not taking any of the numerous good opportunities to cut the enemy's unguarded lines of communication, and for routing frequently. He remarked: "I am disgusted; all my boyhood's fancies of the soldiers of the great Napoleon have been dissipated, or else the soldiers of the "Little Corporal" have lost their elan in the pampered parade soldiers of the 'Man of Destiny'." In 1871, Sheridan was present in Chicago during the Great Chicago Fire and coordinated military relief efforts. The mayor, Roswell B. Mason, to calm the panic, placed the city under martial law, and issued a proclamation putting Sheridan in charge. As there were no widespread disturbances, martial law was lifted within a few days. Although Sheridan's personal residence was spared, all of his professional and personal papers were destroyed. When Chicago's Washington Park Race Track organized the American Derby in 1883 he served as its first president. Sheridan served as commander in chief of the Military Order of the Loyal Legion of the United States (MOLLUS) veterans' organization from 1886 to 1888. He was also the first president of the Society of the Army of the Potomac when it was founded in 1869. On November 1, 1883, Sheridan succeeded General William T. Sherman as Commanding General of the U.S. Army, and held that position until his death. He was promoted on June 1, 1888, shortly before his death, to the rank of General in the Regular Army (the rank was titled "General of the Army of the United States", by Act of Congress June 1, 1888, the same rank held earlier by Grant and Sherman, which is equivalent to a five-star general, O-11, in the modern U.S. Army).

Answer this question "what happened in the franco prussian war" by extracting the answer from the text above.
As a guest of the King of Prussia, he was present when Napoleon III surrendered to the Germans,