It was designed as a forceful response to Japanese assertiveness. But Beijing’s creation of an air defense zone may have backfired, experts said, eliciting a strong joint response by the United States and Japan.

Instead of strengthening China’s position, the “air defense identification zone” has unsettled and united its neighbors. It provided Washington with a perfect opportunity to reassure its Asian allies that it remains committed to maintaining stability in the Asia-Pacific region.

On Saturday, the Communist Party government said any noncommercial aircraft entering a broad zone over the East China Sea should first identify itself and warned ominously that failure to do so could provoke “defensive emergency measures” by China’s armed forces. The statement heightened an already tense standoff with Japan over several disputed islets in the area.

But the United States called China’s bluff by sending two warplanes into the zone Tuesday, and Beijing’s response was muted. The Defense Ministry merely said it had identified and monitored the planes, while the Foreign Ministry stressed that the zone was purely defensive and offered to strengthen communications with other regional players to maintain peace and security.

“We hope relevant countries do not make too much of a fuss about it, panic and read too much into it,” spokesman Qin Gang said.

Paul Haenle, director of the Carnegie-Tsinghua Center for Global Policy in Beijing, said the mild reaction was surprising. “It is almost as though they hadn’t anticipated the U.S. response and didn’t know what to do,” he said.

In Chinese eyes, the standoff began in September 2012, when the Japanese government purchased three of the islands — known as the Senkaku Islands in Japan and Diaoyu in China — from a private Japanese landowner. In response, Beijing stepped up its own claims to the rocky land masses, increasing sea patrols and pressing Japan to accept that the territory is disputed.

Japan, like numerous other countries, already has its own air defense identification zone. The country increasingly has cited the zone as a reason to warn or intercept Chinese planes in the area, according to military experts in Beijing. In September, Japan threatened to shoot down Chinese drones flying over the disputed islands; China warned that such a move would constitute an act of war.

The Chinese military had been considering establishing its own air defense zone for some time, and this increased tension may have tipped the balance, experts said.

“Japan has been acting more and more confrontational with regards to the Diaoyu islands, so China had to roll out its own measures to balance it out,” said Zhou Yongsheng of the Center for Japanese Studies at the University of International Relations in Beijing. “Whenever Chinese aircraft entered Japan’s zone, they would dispatch fighter jets to intercept us, which put us in a very passive position.”

Popular sentiment in Japan and China has also become increasingly hostile toward the other country. Rising nationalism in China is now coupled with genuine concern about the intentions of a more nationalist Japanese government under Prime Minister Shinzo Abe. His government has raised military spending since coming to power in December.

Zhu Feng, a professor of international studies at Peking University, said China had established the air zone to “curb the arrogance of the Abe administration.” Zhao Chu, a military affairs columnist, said China was responding to Japan’s more aggressive enforcement of its own zone.

“But the primary cause is that China needs a stronger foreign policy at present because an appeal for that has been gathering momentum in Chinese society for over a decade,” Zhao said. “It’s a hawkish voice that now dominates Chinese society.”

Beijing’s actions appear to fit into a recent pattern, experts said. Reluctant to be seen as the initial provocateur, China tends to respond forcefully to what it sees as any provocations from others and to then advance its own claims even more strongly.

But China may have overplayed its hand with Saturday’s Defense Ministry announcement, experts said.

Both Secretary of State John Kerry and Defense Secretary Chuck Hagel strongly criticized the move. Vice President Biden will also convey the administration’s concerns when he meets separately with the leaders of Japan, China and South Korea on a weeklong trip to Asia starting Sunday, officials said.

Under President Barack Obama, the United States has announced a “strategic rebalancing” — or pivot — toward Asia, a move that many in China see as an attempt to contain its rise. But when Obama, consumed by the federal government shutdown, failed to attend two important summits in Asia last month, doubts surfaced about the depth of the U.S. commitment. Biden will try to reassure the leaders of Japan and South Korea during his trip, but this week’s events may already have done the job for him.

Hagel called his Japanese counterpart, Itsunori Onodera, on Wednesday, described China’s move as “a potentially destabilizing unilateral action” and commended the Japanese government “for exercising appropriate restraint,” spokesman Carl Woog said in a statement.

Acting on instructions from their government, Japan’s two biggest commercial airlines announced that they would not relay plans to China before flying through its newly declared zone. Onodera told reporters Wednesday that Self-Defense Forces jets would continue to operate in the East China Sea, without reporting their flight plans to Beijing.

“This announcement and quick U.S. response have clarified the U.S.’s siding with Japan much more than the past,” said Mathieu Duchtel of the Stockholm International Peace Research Institute in Beijing. “So there is some degree of miscalculation” by China.

Japan has voiced concern for years about what it sees as China’s increasing maritime aggression, but in recent days those concerns have been echoed by Taiwan and South Korea. The anger from South Korea is particularly notable, because Seoul, under President Park Geun-hye, has drawn closer with Beijing while sparring with Japan over historical issues.

On Wednesday, South Korea’s foreign minister, Yun Byung-se, said in a speech that China’s announcement had “made already tricky regional situations even more difficult to deal with.”

In the past few months, China had been working hard to reassure many of its Asian neighbors that its rise not only did not threaten them but would also actually be to their benefit. China’s leaders have made successful visits to Malaysia, Indonesia and Vietnam and offered billions of dollars in trade and investment. Now, those reassurances may look less convincing.

“This will inject a large dose of anxiety and uncertainty into countries in the region about China’s rise,” said Haenle of the Carnegie-Tsinghua institute.

The episode also appears to underline China’s sometimes confusing foreign and strategic policy stance, with the armed forces often adopting more hawkish positions than the Foreign Ministry. That, said Haenle, was precisely why President Xi Jinping announced this month that he would establish a new high-level committee to better coordinate national security policy.