the U.S. territories was beyond it. In Panama City, if there were no protests interfering with movement, one could enter the Zone simply by crossing a street. Tensions and the end of the Canal Zone In 1903, the United States, having failed to obtain from Colombia the right to build a canal across the Isthmus of Panama, which was part of that country, sent warships in support of Panamanian independence from Colombia. This being achieved, the new nation of Panama ceded to the Americans the rights they wanted in the Hay–Bunau-Varilla Treaty. Over time, though, the existence of the Canal Zone, a political exclave of the United States that cut Panama geographically in half and had its own courts, police, and civil government, became a cause of conflict between the two countries. Major rioting and clashes occurred on May 21, 1958, and on November 3, 1959. Demonstrations occurred at the opening of the Thatcher Ferry Bridge, now known as the Bridge of the Americas, in 1962 and serious rioting occurred in January 1964. This led to the United States easing its controls in the Zone. For example, Panamanian flags were allowed to be flown alongside American ones. After extensive negotiations, the Canal Zone ceased to exist on October 1, 1979, in compliance with provisions of the Torrijos–Carter Treaties. In 1989, the United States invaded Panama and virtually all of the military operations took place within the Canal Zone including Operation Acid Gambit, the Raid at Renacer Prison among many others including operations at the entrance, exit and all of the locks. Lifestyle of residents "Gold" roll and "silver" roll During its construction and into the 1940s, the labor force in the Canal Zone (which was almost entirely publicly employed) was divided into a "gold" roll (short for payroll) classification, and a "silver" roll classification. The origins of this system are unclear, but it was the practice on the 19th-century Panama Railroad to pay Americans in US gold and local workers in silver coin. Although some Canal Zone officials compared the gold roll to military officers and the silver roll to enlisted men, the characteristic that determined on which roll an employee was placed was race. With very few exceptions, American and Northern European whites were placed on the gold roll, and blacks and southern European whites on the silver roll. American blacks were generally not hired; black employees were from