TOKYO -- Ten years ago, when Chinese navy ships were spotted in waters between Japan and China, a newly elected lawmaker named Keizo Takemi warned that the Chinese were surveying energy resources also claimed by Japan. He was ignored by senior colleagues, who said they wanted to keep smooth ties with Beijing.

This year, as China prepares to drill for natural gas below that same part of the East China Sea, Japan is reacting very differently. Mr. Takemi, now a leader on foreign affairs in parliament, put together a response that was surprisingly robust by Japanese standards: In March, Tokyo announced it will launch a rival drilling effort, to be protected by Japan's large, high-tech military if necessary.

"Our nation's sovereign rights are at stake," says Mr. Takemi, 53 years old.

Relations between Japan and China are at their most tense in decades. One cause has been Chinese outrage over visits by Japanese Prime Minister Junichiro Koizumi to a religious shrine that commemorates Japan's war dead, including people convicted of war crimes against China and other Allied countries. In Japan, there has been rising anxiety about China's growing economic and political strength. But behind the scenes in Tokyo, another factor has also altered ties between Asia's two biggest powers: the emergence of a new generation of leaders with new notions about Japan's role in the world.

These younger lawmakers, most in their 40s and 50s, want their nation to be more assertive. They are also willing to break old taboos about shows of military force, something Japan long avoided for fear of conjuring memories of World War II aggression. That is a big change from their predecessors, who avoided confrontation with China, instead showering it with billions of dollars in development aid out of guilt for Japan's brutal 1930s invasion. Memories are still raw in China of the Japanese attack, which historians say caused fighting and famine that killed millions of Chinese civilians.

The younger lawmakers in the Diet, Japan's parliament, are more likely to view China as a rival than a former war victim. Many have vowed to end what they see as their nation's traditional kowtowing to China. They have almost shut off the aid spigot, and they are pushing Japan to respond more aggressively to perceived slights from Beijing.

Experts say the generational change could eventually lead Japan to shed its traditional passiveness in its dealings with the rest of the world as well. Many younger lawmakers say they want their country to be a more active partner of the U.S., even in military operations like Iraq.

"The younger generation is more willing to voice nationalist ideas," says Michael Auslin, a professor specializing in Japanese diplomacy at Yale University. "They don't want to come in second to China, though they still haven't figured out how to take a leadership role."

The immediate result has been a willingness to challenge China that would have been unthinkable just a decade ago. When a Chinese nuclear attack submarine entered Japanese waters last fall, Tokyo chased it with destroyers and aircraft in one of its biggest military operations since World War II. When China announced it wanted to help build a pipeline to buy Russian oil, Japan suddenly offered Moscow a richer deal to try to win the oil for itself. In April, as Chinese protesters stoned Japanese businesses amid demands that Japan apologize for its 1930s invasion, Tokyo replied with demands of its own that China apologize and pay for the damages.

These more assertive gestures have proven popular with voters here, who in recent years have shifted noticeably toward the right and a more nationalistic stand in foreign affairs. A big reason for the shift is fear and envy of China, whose roaring export engine seems to be stealing jobs at a time of economic uncertainty for Japan.

The amount of Chinese-made goods flowing into Japan has tripled since 1995, a time when Japan's overall growth was flat because of a stubborn banking crisis and glut in factory capacity.

The younger lawmakers say they don't want to paint China as an enemy, but want to redefine the relationship to reflect the realities of China's rapid economic and political emergence. Unlike their predecessors, who left the details of foreign policy to the mandarins in Japan's powerful bureaucracy, these younger lawmakers are taking policy into their own hands, drawing up strategies and legislation. Also unlike their predecessors, who followed a traditional career path through Japan's education system, many hold graduate degrees from top U.S. universities, and have adopted a more outspoken American political style.

"A new breed has appeared in the Diet," says Ichita Yamamoto, a 47-year-old upper house member who studied under former U.S. Secretary of State Madeline Albright while earning a master's degree in foreign policy at Georgetown University in the early 1980s. "We have no illusions about China. China is an economic opportunity, but it is also a threat."

Mr. Yamamoto has emerged as a vocal advocate of a changed approach toward China. A thin, energetic man with a boyish face, he exemplifies the new style of politics in Tokyo. In 2003, he released his own CD, titled "Reform Songs," on which he sings rap songs, including one urging Japan to overcome the past and build a new relationship with the rest of Asia. "A call to reform that I want to heed!" says an endorsement on the CD's cover by Prime Minister Koizumi.

"Japan has been too wishy-washy in foreign affairs," says Mr. Yamamoto. "It's OK to clash with other countries when necessary."

When he was first elected to the Diet in 1995, the older generation of parliamentarians left policy-making largely to the Foreign Ministry, which was dominated by "China School" diplomats who had built their careers on maintaining wrinkle-free ties with China. The ruling Liberal Democratic Party's main policy-making forum, a committee of lawmakers called the Foreign Affairs Division, was a sleepy affair in which ministry diplomats showed up to explain their policies to a half-dozen Diet members.

Then, starting in the late 1990s, a series of spats erupted with China, including one in 2002 when Chinese military police entered a Japanese consulate to seize North Korean defectors. That served as a wake-up call to the Japanese public about China's rise. More and more young lawmakers grew critical of traditional China policy as voters in their districts voiced anger at Chinese actions, lawmakers say.

By 2000, debates were heating up at the LDP's foreign-affairs committee, held about once a week at party headquarters in central Tokyo during the fall budget-writing season, say Mr. Yamamoto and others. Soon, the room was packed with as many as 60 lawmakers, many forced to stand, as they faced half a dozen Foreign Ministry bureaucrats seated behind a table.

One hot-button issue was development aid to China, which has totaled 3.34 trillion yen, or $30.6 billion, since it started in 1979. A growing number of lawmakers called for halting the aid, saying it was being used to strengthen a rising economic rival and potential military threat. They also pointed out that China's incomes were on the rise -- though they remain just a fraction of Japan's -- and that China had started giving money to other poorer countries. But bureaucrats and senior LDP members warned that China was still a developing country and cutting aid would hurt ties.

Mr. Yamamoto and others say the tide began to turn in their favor as younger lawmakers heavily outnumbered older ones. He recalls one meeting last fall when it was clear the consensus of opinion among lawmakers in the room, and in the ruling party as a whole, had swung in favor of ending most aid to China.

"Why do we need this for a country that can put up satellites?" one lawmaker demanded of the Foreign Ministry officials present.

Mr. Yamamoto says he warned the bureaucrats against opposing the lawmakers: "This is a political decision. Don't try to block us."

In March, the LDP voted to phase out most aid to China by 2008, when Beijing will host the Summer Olympics.

"Those who opposed development aid to China became more vocal and ended up swaying the others against us," said one Foreign Ministry official who was present.

The Foreign Ministry also lost its staunchest supporters with the disappearance of the older generation of lawmakers. Now in their 60s and 70s, many have retired. Others have fallen victim in recent years to a political fight within the ruling party, as Prime Minister Koizumi has steadily gained in a struggle for control of the LDP against the old guard.

The dwindling number of older LDP politicians has been left fuming on the sidelines. One is Takeshi Noda, 63, a second-generation lawmaker who fought to continue aid to China, which he said was still needed to assuage Chinese anger over the war.

Mr. Noda, who first joined the Diet in 1972, the year Japan established diplomatic ties with China's Communist government, says he feels a personal stake in China relations. His father, former Diet member Takeo Noda, played a key role in re-establishment of relations with Beijing. The elder Mr. Noda held talks with Chinese leaders Mao Zedong and Zhou Enlai, including a secret trip to China during a 1959 border war between China and India to try to broker peace, his son says. The younger Mr. Noda says he, too, has made several trips to China, the first in 1974.

"We had to overcome a deep resentment of Japan because of the war," Mr. Noda says. "In Chinese, there is a saying: doing something hard is like filling a well. My father's generation and my generation filled that well." He blames Mr. Koizumi for leading a nationalist turn among lawmakers, whom he says are simply pandering to public opinion.

Mr. Koizumi has helped set an antagonistic tone toward China by insisting on making annual visits to Yasukuni Shinto shrine, a sprawling memorial in central Tokyo honoring Japanese soldiers fallen in wars since Japan emerged from feudalism in the mid-19th century. China, South Korea and other former victims of Japanese aggression complain loudly because Yasukuni also honors the souls of war criminals executed by the Allies after World War II. Outrage over Mr. Koizumi's annual visits to Yasukuni was one cause of April's anti-Japanese riots in Chinese cities. But Mr. Koizumi persists, partly because the visits have proven popular among conservative voters, who make up a core support group of the LDP.

One goal of the new generation of lawmakers has been to wrest control of policy from bureaucrats in the Foreign Ministry. Among the most successful has been Mr. Takemi, who has emerged as a leading voice in the new line toward China. Mr. Takemi's pet issue has been the East China Sea, between Shanghai and the Japanese island of Okinawa, where both countries make overlapping claims to economic rights.

Mr. Takemi says it wasn't until two years ago that his ideas started to get attention in the LDP. In 2003, Mr. Takemi created a working group of 26 LDP lawmakers to formulate policy recommendations. To lessen dependence on China experts in the Foreign Ministry, Mr. Takemi, himself a former professor of international affairs who was a visiting scholar at Harvard University, added a staff of three university and think-tank researchers.

He says the real wake-up call came last year, when China started building an offshore platform in disputed waters to drill for natural gas. The platform will start extracting natural gas as early as this summer, says builder China National Offshore Oil Corp. Japan dispatched its own survey ship to the area, which Chinese ships tried to block. Interest in Tokyo was suddenly so high that in March, Mr. Takemi led a multipartisan group of-- young lawmakers on an inspection of the East China Sea aboard a Japanese Coast Guard jet. "When we saw how huge the Chinese drilling platform was, we realized we needed to act right away to start drilling, too," Mr. Takemi says.

A week later, he guided the drafting of a three-page "emergency proposal" that recommended Japanese drilling. The recommendation won quick endorsement by the Koizumi Cabinet, which ordered the trade ministry to act on it. Beijing has criticized Japan's actions, saying they infringed upon China's sovereignty.

"In the old days, the bureaucrats drew up policy," Mr. Takemi says. "Now, we make the decisions, and they have to implement them."