The rumbling you heard across the Pacific Ocean over the weekend was Japan moving further from the United States -- and closer to China. Japan's left-of-center opposition party, which has long spouted anti-capitalist and anti-US rhetoric, won a landslide victory in Sunday's election.

The Democratic Party of Japan wound up with 308 seats in the powerful lower house of Parliament, trouncing the ruling right-of-center Liberal Democratic Party, which won just 119 seats. It's only the second time in 54 years that the LDP has been out of office.

While historic, the result was no surprise. Japanese voters, angry at the government's inability to fix longstanding economic problems, were determined to drive out incumbents -- a case of "better the devil you don't know."

How will this affect the US-Japan alliance? No one knows -- not even the DPJ.

The new ruling party is a big tent covering socialists who advocate pacifism as well as conservatives who favor expanding Japan's global security role, as the United States has asked. How far the new government shifts Japan's policies depends largely on which faction sets the party's agenda.

But Tokyo will certainly be less willing to fulfill existing defense agreements and more resistant to Washington's future security requests. A poll of DPJ candidates showed that only a minority supported such US-friendly moves as dispatching Japanese forces to Afghanistan and continuing to refuel US ships in the Indian Ocean to help the global War on Terror.

The DPJ has long called for a foreign policy more independent of Washington. Prime Minister-elect Yukio Hatoyama praises the US-Japan alliance in broad terms -- but actually promotes a more Asian-centric strategy for Japan. The apparent aim is to create a politically and economically integrated regional bloc, similar to the European Union.

Ichiro Ozawa, the DPJ's second-in-command, has consistently long argued for downplaying the US alliance in favor of deploying Japanese forces only on UN-sanctioned missions.

The Obama administration will have its hands full balancing the pursuit of US security objectives with maintaining strong relations with a pricklier Japanese ally. More than ever, Washington's ability to influence Tokyo's policies will be hindered by a ruling party skeptical -- if not suspicious -- of US intentions.

Japan is critical to many US strategic objectives, including maintaining peace in the region. Washington must call for the DPJ to affirm the existing alliance and the bilateral policies that flow from it. It should also press Tokyo to stick by its commitments, particularly on Indian Ocean refueling, missile defense and US military redeployment in Japan.

Even under the LDP, Tokyo preferred to emphasize economic solutions to security challenges and maintained only limited means to project military power, even to defend its national interests. The DPJ will be an even greater test of US patience.

Along with its constrained armed forces, Japan's economic woes and political paralysis have been diminishing Tokyo's regional and global influence. Meanwhile, China's growing economic and military capabilities increasingly enable Beijing to fill the gap.

Perhaps the best the Obama administration can hope for with the new Japanese government is maintaining the bilateral status quo. Yet that relationship is increasingly inadequate to address either Asian or global security challenges. Absent more help from Japan, the US military may at some point be unable to meet ever-growing demands in the region.

If Japan won't pull its weight, expect greater crises in Asia and around the world.

Bruce Klingner, is senior research fellow for Northeast Asia in the Asian Studies Center at the Heritage Foundation.