After a year of scandal and missteps, the prime minister of Japan, Shinzo Abe, resigned yesterday. The abrupt announcement came just days after Mr. Abe pledged to fight for renewal of a law allowing Japanese naval ships to aid coalition forces in Afghanistan.

So what made the prime minister resign?

The proximate cause was plummeting public support. Mr. Abe made disastrous personnel choices for certain cabinet positions, notably the agriculture minister. Allegations of graft and misuse of funds led one of his ministers to commit suicide earlier this year.

Overall, five of the prime minister’s cabinet appointees resigned. On top of that, Mr. Abe never successfully resolved the ongoing pension scandal, in which over 50 million pension records were lost.

But perhaps most damningly, even after he refused to resign last month and formed a new cabinet, Mr. Abe brought on replacement ministers who had to resign within days due to fresh corruption allegations. He had squandered his final chance.

Mr. Abe, part of the Liberal Democratic Party, was pushed toward his decision by the unbending tactics of Ichiro Ozawa, the head of the Democratic Party of Japan, the main opposition party. Mr. Ozawa left the LDP, the ruling party, in 1993 after alienating many of the older party bosses who objected to his aggressive tactics. It was Mr. Ozawa’s desire for power that spurred the beginnings of the twoparty system in Japan, as various splinter groups and disaffected LDP politicians began coalescing around him. Yet Mr. Ozawa was never able to form an outright majority and, until recently, had been seen as a spent force in Japanese politics. Back in the 1990s, Mr. Ozawa championed a more assertive foreign policy for Japan: one not as subservient to America, and yet one that gave Japan a global role commensurate with its economic power. But since the July upper house poll, he has refused to consider an extension of the anti-terrorism law that allowed Japanese maritime forces to provide thousands of gallons of fuel to coalition ships operating near Afghanistan. This was a puzzling move, since the election had focused on domestic issues. Mr. Abe, however, had declared his intent to get the renewal passed, otherwise Tokyo would have to withdraw its ships from the Indian Ocean starting on November 1.

As Mr. Ozawa revealed his intransigence, the government stepped back, eventually floating an idea to scrap the current law and pass a measure focused more on offering humanitarian aid to the Afghans. Mr. Abe then tried to rally by suggesting he would seek a lower house override of the upper house’s refusal to extend the mission. Mr. Ozawa pounced on this, and rumors that the upper house would censure Mr. Abe began spreading around Tokyo.

In a last ditch effort, Mr. Abe indicated he wanted to meet with Mr. Ozawa to hammer out a compromise, but Mr. Ozawa refused. Facing no options, Mr. Abe made a snap decision to resign.

Such “inside baseball” politics may be of little interest to American observers, who are more concerned with what effect this will have on U.S.-Japanese cooperation. Such concerns are warranted, though perhaps not for the reasons Americans think .

Privately, some American government officials are furious with Mr. Ozawa for playing politics with the anti-terrorism law. But they are equally furious with Mr. Abe for his helplessness. The Japanese have supplied millions of dollars worth of high-grade fuel, which the U.S. cannot deliver, to Pakistani and British ships. The fear is that Pakistan will withdraw its ships if the Japanese leave, thereby eliminating the one Muslim nation in the Afghan coalition.

For this reason, Japan’s role appears crucial to U.S. policymakers. But they are much less focused on its impact in Japan. Clear majorities of the Japanese public don’t support the refueling mission, in part because Mr. Abe did a poor job selling it.

More importantly, Mr. Ozawa and others have made the case that Japan’s subservience to America is both unconstitutional and dangerous — that it will drag Japan into faraway conflicts of little immediate consequence to Japanese national interests.

Mr. Abe’s plan to revise Article 9 of the Japanese constitution, which currently prevents Japan from engaging in collective self-defense, lies in tatters, and Tokyo may be less inclined to join overseas operations in the near future.

This is the result of the globalization of the U.S.-Japan alliance. Washington’s desire for a reliable, go-to partner may have made strategic and even tactical sense, but it appears that Messrs. Koizumi and Abe’s willingness to fill that role put them far ahead of their fellow citizens.

Neither Mr. Koizumi nor Mr. Abe made a particularly compelling case for why it was in Japan’s best interests to dramatically expand its international activities, especially when those activities required military forces. Some folks in Washington and Tokyo even allege that Japan joined the war on terrorism solely as a down payment for American support against North Korea’s nuclear programs. Mr. Ozawa now is insisting that Japan only join U.N.-run operations, and that it put less emphasis on the U.S. alliance. This is a risky ploy, both diplomatically and electorally. Washington is unlikely to significantly alter alliance promises, but a more independent approach on Tokyo’s part may, if maintained long enough, slowly drive the two partners apart, as was the case with America and France during the Cold War.

If Japanese voters get a chance to see what such isolation is like, they may well punish Mr. Ozawa for doing unnecessary damage to Japan’s most important alliance. In either case, it will have an impact on Washington’s dealings with the rest of Asia, where some believe America is moving closer to China.

Japan is now at a crossroads. Mr. Abe’s likely successor, the foreign minister, Taro Aso, does not have deep political support among the populace, and is suspected by Japan’s neighbors of being a hardliner.

Mr. Aso — or whoever becomes premier — faces serious challenges. Will economic reform continue? Will Japan’s bold, yet so far largely rhetorical, new diplomacy survive? Is this the beginning of a true two-party system in Japan? Will Washington give its Asian ally the space to sort out these developments? There’s a lot more riding on it than free gasoline.

Mr. Auslin is a resident scholar at the American Enterprise Institute. This piece was adapted from the online edition for the American magazine.