One of the first fire support bases constructed by U.S. troops was built in October 1965. Designated Bill, it was built by the First Cavalry Division in Pleiku Province soon after the division arrived in South Vietnam. Based on the original concept developed by that division, firebases would move about every two days, minimizing the amount of security and semi-permanent construction they would need. Over time this changed, and firebases evolved into small forts with all the defensive measures those required.