TOKYO A feud over plans to relocate a U.S. military base on Japan's Okinawa island as part of a broad reorganisation of U.S. troops has strained Washington's ties with Tokyo ahead of President Barack Obama's Nov. 12-13 visit.
The row coincides with deepening questions about how China's rising military and economic clout will reshape the decades-old U.S.-Japan alliance, long seen as vital to regional security.
Below are some questions and answers about the origins of the dispute and whether an alliance crisis can be avoided.
WHY CLOSE THE FUTENMA BASE AND REPLACE IT?
Residents of Okinawa, 1,600 km (1,000 miles) south of Tokyo and reluctant host to about half the 47,000 U.S. military personnel in Japan, have long resented what they see as an unfair burden in maintaining the U.S.-Japan security alliance.
The concentration of U.S. military bases on Okinawa, a major U.S. military forward logistics base in the western Pacific, is a legacy of America's occupation of the island from 1945 to 1972.
Many locals associate the bases with crime, noise, pollution and accidents, and outrage flares periodically -- most strikingly after the 1995 rape of a schoolgirl by three U.S. servicemen.
As part of a 1996 pact to reduce the U.S. military presence, the United States and Japan agreed to close Futenma Air Station, home to about 4,000 Marines and located in crowded Ginowan City, within seven years if a replacement could be found in Okinawa.
An initial plan for an off-shore facility in northern Okinawa was opposed by locals and environmentalists.
The current plan is for relocation to a northern site to be partly built within another U.S. base and on reclaimed land.
IS THIS JUST ABOUT FUTENMA?
No. The issue is much broader. Washington and Tokyo agreed in 2006 on a "road map" to transform the decades-old alliance, the pillar of Japan's post-World War Two security policies.
Part of a U.S. effort to make its military more flexible globally, the realignment fit with efforts by Japan's then-ruling Liberal Democratic Party (LDP) to shed the constraints of its postwar pacifist constitution and assume a higher global security profile.
Central to the pact was a plan to reorganise U.S. troops in Japan, including a shift of about 8,000 Marines by 2014 to the U.S. territory of Guam from Okinawa. The Marines' move, however, depends on finding a replacement site for Futenma.
While Futenma and the Marines' move grab most headlines in Japan, a package of steps to improve U.S.-Japan military cooperation in areas such as missile defence is equally vital.
WHY IS THIS DISPUTE FLARING NOW?
Prime Minister Yukio Hatoyama's Democratic Party of Japan, which took power in September, promised in the campaign leading up to its election victory in August that it would review the realignment pact as well as the Status of Forces Agreement (SOFA) governing the U.S. military in Japan. Hatoyama had said moving Futenma's functions off Okinawa was best.
More broadly, the Democrats have promised to adopt a diplomatic stance less subservient to its close security ally Washington, a shift from the long-dominant LDP, which was defeated in the election.
U.S. Secretary of Defense Robert Gates turned up the heat last month when he stated bluntly that the deal must be carried out. But State Department spokesman Ian Kelly said on Wednesday that the United States had not set a deadline.
Anxiety is being exacerbated by questions about the overall future of the five-decade-old U.S.-Japan alliance as both face the challenge of China's rising economic and military might.
Some in Japan fear Washington will cosy up to Beijing, while some in the United States worry Tokyo is tilting towards Asia by promoting the idea of an East Asia Community trading bloc.
CAN A CRISIS BE AVOIDED?
The best bet is for the two sides to turn down the heat and delay a resolution until after Obama's visit. Tokyo would need to convince Washington that it doesn't plan to delay too long.
The United States is unlikely to agree to reopen talks given its firm public rejection of that option and Obama's need to focus on other pressing issues such as the war in Afghanistan.
Japan might eventually agree to the current plan as is, or with slight modifications, but bowing to U.S. demands could cause a rift with two tiny coalition partners whose backing is needed to pass laws smoothly, as well as within the Democratic Party.
Appearing to dither or to endanger the U.S.-Japan alliance could undermine Hatoyama's public support, but caving in suddenly to U.S. pressure could also alienate some voters.
WILL THIS AFFECT ECONOMIC TIES?
Few analysts expect bilateral strains to spill over into trade and investment ties between the world's two biggest economies, and financial markets have taken the row in their stride even as market players express concern.
But damage to U.S.-Japan ties could spell geopolitical uncertainty in a region home to a rising China and an unpredictable North Korea, eventually affecting investment flows.