SEOUL, South Korea  When Prime Minister Yukio Hatoyama of Japan abruptly stepped down Wednesday, largely for his failure to move an American air base off Okinawa, he was essentially admitting he had not won popular support for a prominent campaign pledge: ending Japan’s postwar dependence on the United States for its security.

“This has proved impossible in my time,” Mr. Hatoyama said in a teary speech to explain his decision to step down. “Someday, the time will come when Japan’s peace will have to be ensured by the Japanese people themselves.”

Mr. Hatoyama’s plunge in popularity, just eight months after his victory ended a half-century of nearly unbroken one-party control, suggested that the Japanese public had rejected his attempt to rethink Japan’s cold-war-era alliance with the United States, its most important ally. Rising tensions between North and South Korea in recent weeks and an increasingly assertive China reinforced the public’s sense that Japan needed the United States more than ever.

The Obama administration’s reaction to the resignation suggested that it would not miss Mr. Hatoyama much either. The White House, in its statement, pointedly did not thank or praise him, saying only that the alliance would “continue to strengthen,” regardless of who was in charge. Senior officials often seemed frustrated by his decision-making, and President Obama never developed a rapport with him.