U.S. Secretary of State Hillary Clinton is expected to play down a row over a U.S. airbase that has frayed ties with close ally Tokyo and eroded support for Japan’s prime minister when she visits Japan on Friday.

The feud has distracted the allies as they try to cope with an unpredictable North Korea and a rising China, while voter perception that Prime Minister Yukio Hatoyama has mishandled the issue is eroding support before a mid-year election his party needs to win to avoid policy paralysis.

Following are some questions and answers about the issue:

WHY HAS THIS DISPUTE COME TO A HEAD NOW?

In the election that swept his Democratic Party of Japan (DPJ) to power last year, Hatoyama raised hopes on the southern island of Okinawa that the Marines’ Futenma airbase could be moved elsewhere, despite a 2006 deal to shift it to a less crowded site on Okinawa, host to about half the 49,000 U.S. military personnel in Japan.

Hatoyama has set himself an end-of-May deadline for resolving the issue, and said he would stake his job on meeting it.

But with no new deal in sight Hatoyama has changed tack, saying some Marines would have to stay in Okinawa to deter threats, a shift that outraged many Okinawans and upset a small ruling coalition partner, the Social Democratic Party (SDP).

The Democrats have also promised to take a diplomatic stance more independent of Washington, but talks on reviewing the five-decade-old alliance have been snarled by the Futenma feud.

CAN HATOYAMA STAY ON AFTER DEADLINE?

Hatoyama has been trying to redefine what “resolving” the row means and appears to be putting priority on reaching agreement with the United States.

Domestic media say the two governments will announce on May 28 an agreement to stick to the 2006 plan with minor changes.

That risks outraging many Okinawans, irking the DPJ’s coalition partner and leaving voters wondering what the fuss was all about.

The tiny Social Democratic Party’s votes are no longer needed to pass bills smoothly in parliament after some upper house lawmakers switched sides, but a rift in the coalition ahead of an upper house election expected on July 11 would be ill-timed.

Analysts say Hatoyama will likely stay on despite the fuss, partly because the Democrats had criticised two predecessors from the rival Liberal Democratic Party for quitting after only a year and because time is running out before the upper house poll.

The dispute seems unlikely to spill over into trade and investment ties between the world’s two biggest economies. Trade between the United States and Japan amounted to 14.2 trillion yen ($159 billion) in 2009, while two-way flows between China and Japan totalled 21.7 trillion yen.

But damage to the alliance could create uncertainty in the region, eventually affecting investment flows.

WHY CLOSE THE FUTENMA BASE AND REPLACE IT?

Residents of Okinawa, 1,600 km (1,000 miles) south of Tokyo and the site of a bloody World War Two battle, resent what they see as an unfair burden for maintaining the security alliance.

Outrage flares periodically among residents over accidents, crime and pollution associated with the bases — most strikingly after the 1995 rape of a schoolgirl by three U.S. servicemen.

For the U.S. military, Okinawa provides a forward logistics base strategically located in the western Pacific close to Taiwan and the Korean peninsula.

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 2,000 Marines and located in crowded Ginowan City, within seven years if a replacement could be found on Okinawa.

An initial plan for an offshore facility in northern Okinawa was opposed by locals and environmentalists. The 2006 plan would shift the facility to the northern city of Nago, where it would be partly built within another 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 efforts by the then-ruling Liberal Democratic Party to shed the constraints of Japan’s pacifist constitution and assume a higher security profile.

Central to the pact was a plan to reorganise U.S. troops in Japan, including a shift of up to 8,000 Marines by 2014 to the U.S. territory of Guam from Okinawa. The Marines’ move depends on finding a replacement site for Futenma, although some critics have questioned whether the two really need to be linked.

(Additional reporting by Isabel Reynolds and Chisa Fujioka; Editing by Paul Tait)