President Barack Obama hopes to build on his new friendship with Indian Prime Minister Narendra Modi during his upcoming trip to India.
President Barack Obama hopes to build on his new friendship with Indian Prime Minister Narendra Modi during his upcoming trip to India.
(CNN) —  

On a sunny morning in September, Indian Prime Minister Narendra Modi arrived at the White House facing low expectations for renewed U.S.-India relations and awkward questions about his visa status in the United States.

A few hours later, Modi was taking a previously unplanned tour of the Martin Luther King Jr. monument on the National Mall with President Barack Obama as his personal guide.

READ: What didn’t make the State of the Union

The quiet moment was a rare incident of spontaneity on Obama’s otherwise rigid daily agenda, and as the leaders gazed at the stone memorial in the midday sun, White House aides say a personal chemistry between the two men began to form.

What came afterward was a surprise to everyone.

At a November meeting of Asian leaders in Burma, the Indian prime minister delivered Obama an astounding request: become the first American president to headline India’s Republic Day parade.

Obama agreed, and on Monday, he will stand alongside Modi to take in the colorful yearly spectacle marking the emergence of the world’s largest democracy.

The invitation, Obama’s aides say, came as a shock - but also presented a diplomatic opening unseen between the two countries in decades.

“It took us by some surprise,” said Ben Rhodes, the President’s deputy national security adviser. “I think (Obama) sees this as a potentially transitional, if not transformational, moment for the relationship.”

SEE ALSO: Why Obama’s trip to my homeland is meaningful

Obama’s objective when he lands in New Delhi on Sunday: translate the personal warmth he’s developing with India’s leader into diplomatic progress that’s eluded the United States in India before.

“There’s a great affinity between the United States and India and our people, but there’s also a history that is complicated,” Rhodes said.

Complicated is a mild way to describe the often tense relationship between the two countries, at odds on trade, climate and defense equipment for years. Analysts say most of those tensions are likely to persist, despite early efforts of Modi’s administration to foster closer bonds to the United States.

Longstanding concerns in India about close U.S. relations to regional rival Pakistan have also persevered, even as Washington’s relationship with Islamabad soured after the raid on Osama bin Laden’s compound. With U.S. officials describing the relationship with Pakistan on the “uptick,” Indian worry is growing.

“Concern in India about America’s continued support for Pakistan is growing to a level that I haven’t seen in well-over a decade,” said Rick Rossow, the Wadhwani Chair in U.S.-India Policy Studies at the Center for Strategic and International Studies, noting that the drawdown of U.S. troops in Afghanistan has led to fears among the Indian public that terrorists could cross through Pakistan and into India.

Obama’s visit - the first time a president has traveled to India twice while in office - won’t allay those concerns completely. But his presence at the widely televised Republic Day military parade sends a unifying message, according to experts.

RELATED: Netanyahu visit causes headaches at home and abroad

“I think you will see the President and the Prime Minister make it a point to try to appeal to various kinds of symbolism and the President especially reach out and try to connect with the Indian public,” said Tanvi Madan, the director of the India Project at The Brookings Institution.

In the minds of U.S. and Indian officials, even small agreements can begin to amount to larger progress on developing a better relationship between the world’s two largest democracies.

One area diplomats hope to make progress this week: India’s nuclear liability law, signed in 2010, that essentially prevents American firms from constructing plants in the country. Outside analysts say they expect some progress on easing the measure.

On climate, the White House wants to secure carbon reduction commitments in the model of what Obama achieved in China late last year, though such aggressive commitments aren’t as likely on this trip, experts say.

“We won’t see a China-style deal, partly because India has spent the last few months separating itself away from the Chinese position on this,” said Brookings’ Madan, suggesting agreements on clean energy may be more likely.

Any major breakthrough with India would have been hard to imagine six months ago. Analysts and even some diplomats hadn’t seen huge potential for the Obama-Modi meeting in September, which was clouded by the hurdle of ensuring Modi would be allowed to enter the United States after being denied a visa a decade ago for alleged involvement in sectarian violence.

That Obama is now flying to New Delhi at Modi’s invitation may be a signal of how quickly progress could be achieved on other, more substantive, fronts - if the U.S. President can capitalize on his new friendship.

“What does the President of the United States want? Does he want a best friend, or does he want somebody that’s going to carry through on commitments?” said Rossow. “It’s clearly the latter.”

OPINION: Is India getting nostalgic over Russia ties?