The treaty was signed in Washington on January 19, 1960, and came into effect on June 23 of that year after the treaty was approved by the Japanese parliament - amid a fair amount of public opposition at the time that has since largely faded. Under the pact, some 47,000 US troops are stationed in Japan, more than half on the southern island of Okinawa, where local residents have complained for years about noise, pollution and crime around the bases.
Washington and Tokyo agreed in 2006 to reorganise US troops in Japan, including moving 8,000 Marines to the US territory of Guam, as a way to lighten the burden on Okinawa. Part of that plan involves relocating US Marine airfield Futenma to the northern part of the island where it less congested.
But Hatoyama's government - which came to power in September after trouncing the long-ruling and staunchly pro-US conservatives in general elections _ has balked at moving Futenma to the designated location. Some Cabinet members say they want it moved off Japanese territory entirely, which is the sentiment of many local residents.
Administrations in both countries have been eager to play down the dispute, with Hatoyama repeatedly saying the 50-year old security pact is the "cornerstone" of Japan's diplomacy - even as he has stressed that he wants Tokyo's ties with Washington to be more equal and less subservient.