It was supposed to be a “golden period” in relations between India and China, but it is looking seriously tarnished.

Early this year, China’s top diplomat, Dai Bingguo, arrived in New Delhi for a 15th round of talks between the nuclear-armed neighbors over their long — and long-disputed — border, proclaiming that they shared a historic opportunity to forge a brighter future “hand in hand.”

But already India and China are squabbling again, and their frontier is the flash point.

A visit by India’s defense minister to a border state claimed by China, accompanied by a fly-past by fighter jets recently stationed in the area, provoked some frosty advice from Beijing not to “complicate” matters. In return, the Indian defense minister, A.K. Antony, called China’s comments “most unfortunate” and “really objectionable.”

The spat, experts say, is a symptom of a deterioration in relations that began in 2005, as India drew closer to the United States and negotiated a civil nuclear cooperation agreement.

That new alignment appeared to threaten Beijing and set relations with India on a downward spiral — so much so that India’s multibillion-dollar military-modernization plans are now largely directed toward containing the growing threat from China.

“Ever since the U.S. nuclear deal in 2005, relations with China have been going through a turbulent time,” said Brahma Chellaney at the Center for Policy Research in New Delhi. “Nothing has changed in recent months to suggest that turbulence is easing or subsiding. What we are seeing actually is that Chinese state media is taking an increasingly hard line.”

At the heart of the tension lies a seemingly intractable border dispute that erupted into a brief war in 1962.

China claims the northeastern Indian state of Arunachal Pradesh, a thickly forested, mountainous region that shares cultural links with Tibet. India contests China’s occupation of a barren plateau in Kashmir, far to the west.

In 2005, the two sides agreed to respect “settled populations” in any final deal, suggesting that they might one day agree to accept the status quo. But soon after the U.S.-India nuclear agreement was signed, the backsliding began.

China took every opportunity to reassert its claim to Arunachal, which it refers to as Southern Tibet. Sensing that there was no longer any hope of a deal, India hardened its position, too.

The extent of the deterioration in relations was underlined this week when a team of Indian foreign policy experts and former senior officials warned that India needed to be better prepared in case China decided to assert its territorial claims by force.

“There is the possibility that China might resort to territorial grabs,” they wrote in a major review of Indian foreign policy, saying China probably would aim to occupy “bite-sized” chunks of land along the ill-defined frontier. “We cannot also entirely dismiss the possibility of a major military offensive in Arunachal Pradesh or Ladakh [Kashmir].”

In January, China denied a visa to an Indian air force officer who comes from the state and was due to visit Beijing as part of an Indian military delegation. New Delhi responded by canceling the entire trip.

Antony then visited Arunachal for the state’s silver jubilee celebrations. The festivities included a fly-past by India’s top-of-the-line fighter jets, the Russian-made Sukhoi-30s, pointedly led by the same officer who was denied the visa. The Sukhois were stationed just outside Arunachal last year to counter the Chinese threat.

“India should maintain the peace and safety of the border area together with China and refrain from taking any action that could complicate the issue,” Foreign Ministry spokesman Hong Lei said in Beijing.

It is the sort of diplomatically worded objection that India might have ignored a few years ago but now feels compelled to rebut. “India will not tolerate external interference of China into Indian territorial affairs,” Foreign Minister S.M. Krishna said.

In Chinese state media, calls for restraint and tolerance are mixed with jabs at the Indian government for being “pushy” or “surrendering” to increasingly nationalist public opinion. An article this month in the People’s Daily, a Communist Party mouthpiece, even upbraided India for suggesting that China’s occupation of a slice of Kashmir was in dispute at all.

India is the world’s largest arms importer, and as tensions with China have risen, it has embarked on a military-modernization plan that is expected to cost $100 billion over the next decade.

In January, India selected France’s Rafale for a $15 billion contract to supply 126 new fighter jets, while the air force has been upgrading landing strips throughout the Himalayas.

The army has deployed about 36,000 additional troops near Arunachal Pradesh and plans to raise two more mountain divisions. At the annual Republic Day parade in January, India unveiled its latest and longest-range nuclear-capable missile, able to fly more than 2,000 miles and reach deep into China.

India’s navy has taken a Russian nuclear submarine on a 10-year lease, and it gathered maritime officers from 14 countries for exercises beside its strategically important Andaman Islands in the Indian Ocean, a meeting that conspicuously excluded China. India is also spending $2 billion to set up a military command on the islands to counter China’s growing influence in the region.

“The Indian military is strengthening its forces in preparation to fight a limited conflict along the disputed border and is working to balance Chinese power projection in the Indian Ocean,” James R. Clapper Jr., the U.S. director of national intelligence, told a Senate committee last month.

A full-blown war between India and China appears highly unlikely, but a small border skirmish can’t be ruled out unless the two sides arrest the slide in relations, some experts say. With China’s leadership embroiled in a succession contest and India’s government seen as paralyzed by a lack of leadership, they are pessimistic about the chances of that happening soon.

“The trajectory is all downwards, and there has been no significant attempt to address the issues that matter to both sides,” said Harsh Pant, a lecturer in the department of defense studies at King’s College London. “Before 2006, no one even talked of a Sino-Indian conflict, and economic relations were seen in a much more positive light. But that sense is gone now.

“China is India’s biggest trading partner, but that does not preclude the possibility of some kind of border kerfuffle or minor skirmish in coming years,” he said.

Researcher Zhang Jie in Beijing contributed to this report.