In February, 1839, is sent by the President as special agent to northern Maine, where the State of Maine and the Canadian province of New Brunswick are in arms against each other over a dispute upon the boundary between. Again by his rare good judgment and by his influence with the authorities upon either side, he averts what might easily have resulted in another war. In 1840 he is proposed as the Whig candidate for President, but he declines in favor of General William Henry Harrison, who is elected. June 25, 1841, appointed full major-general. July 5, 1841, appointed chief of the Army, a position that he occupies for twenty years. From 1841 to 1846 is busied with the duties of his office. He aims to enforce justice and discipline among the rank and file. August, 1842, he issues general orders forbidding the practice of officers striking enlisted men and cursing them, and directs that in cases of offense the regulations of the service be employed. In the summer and fall of 1846, believing that the campaign by General Zachary Taylor to conquer Mexico by invasion from the Rio Grande River border cannot succeed, he advises an advance upon the City of Mexico from Vera Cruz on the Gulf. He asks permission to lead the army in person. November 23, 1846, he is directed by the Secretary of War to conduct the new campaign. Leaves Washington for New Orleans November 25. In his absence a bill is introduced in Congress to create the rank of lieutenant-general, and thus place over him a superior officer. This movement for politics was defeated, but General Scott felt that he had “an enemy in his rear.” Under these conditions he goes to meet General Taylor at the Rio Grande in January, 1847, and detaches a portion of the forces for the Vera Cruz campaign. This makes an enemy of General Taylor. February 19, 1847, he issues general orders declaring martial law in Mexico, for the purpose of restraining the Volunteers from abusing the people of the conquered territory. This wins over the natives and restores discipline. March 9 to September 14, 1847, he conducts the campaign by which the City of Mexico, is captured. September 14, 1847, to February 18, 1848, he remains in charge of the military government in Mexico. By his enforcement of martial law that respects the persons and property of the Mexican people he gains the leaders’ confidence. He is proposed for dictator of the Mexican Republic, with a view to annexation to the United States, but declines. February 18, 1848, he receives orders from President Polk to turn over his command to Major-General William O. Butler, and report for trial by a court of inquiry, on charges that he had unjustly disciplined Generals Quitman and Pillow, and Lieutenant-Colonel Duncan. He is acquitted.