what was the objective of doing this?
Water fluoridation is the controlled adjustment of fluoride to a public water supply to reduce tooth decay.
Water fluoridation
Water fluoridation is the controlled adjustment of fluoride to a public water supply to reduce tooth decay. 
when did they start putting fluoride in water 1945
wiki:24481642
Water fluoridation [SEP] Introduction
Water fluoridation is the controlled adjustment of fluoride to a public water supply to reduce tooth decay. Fluoridated water contains fluoride at a level that is effective for preventing cavities; this can occur naturally or by adding fluoride. Fluoridated water operates on tooth surfaces: in the mouth, it creates low levels of fluoride in saliva, which reduces the rate at which tooth enamel demineralizes and increases the rate at which it remineralizes in the early stages of cavities. Typically a fluoridated compound is added to drinking water, a process that in the U.S. costs an average of about $ per person-year.