Over the past year, a pair of legal bombshells have put India’s growing relationship with the United States to one of its biggest tests yet.

Just as the two sides were announcing unprecedented expansions in defense and technology ties, U.S. prosecutors accused Indian government agents of plotting to assassinate an American citizen on U.S. soil.

Months later, the Justice Department filed fraud and bribery charges against India’s most prominent business mogul, whose enterprises have soared to dizzying heights on the back of Prime Minister Narendra Modi’s power.

Still, the relationship has held. After decades of mutual suspicion between the two countries, said Eric Garcetti, the departing U.S. ambassador to India, the fact that now nothing seems to derail their ties is proof of their strength.

“I don’t think there is anything out there big enough to threaten the trajectory of the U.S.-India relations,” Mr. Garcetti said on Saturday in an interview at the embassy in New Delhi, two days before President Biden leaves office and Donald J. Trump is sworn in as his successor.

“This is incredibly resilient and almost inevitable,” Mr. Garcetti added. “It’s really the pace and the progress that’s not inevitable, like how quickly we get there.”

The Biden administration’s doubling down on the relationship with India came after nearly two decades of efforts to shed Cold War-era suspicions that had culminated with U.S. sanctions on India’s nuclear program in 1998.

Washington sees great potential in India as a geopolitical counterweight to an increasingly assertive China. Already the world’s largest democracy, India took over from China as the world’s most populous nation in 2023. India’s demographic advantages and growing technological capacity could help diversify global supply chains away from China, a priority of the United States and other major powers.

Now comes Mr. Trump’s second presidency, with its America-first orientation and threats of steep tariffs on trading partners. While leaders of many countries are unnerved, Indian officials insist that they are not among them.

S. Jaishankar, the foreign minister, has said India enjoyed “a positive political relationship with Trump” that it hopes will only deepen. As he attended the opening of a U.S. Consulate on Friday in the tech hub of Bengaluru, also known as Bangalore, Mr. Jaishankar quoted Mr. Modi as saying that the two countries were overcoming “the hesitations of history.”

Mr. Modi has enjoyed a strong rapport with Mr. Trump, an important factor because of the incoming president’s personal approach to international relations. During Mr. Trump’s first term, Mr. Modi hosted him at a grand rally in his home state of Gujarat, as well as at a large gathering in Texas of the Indian diaspora — an increasingly crucial extension of the Indian influence in American politics.

But some analysts cautioned that Mr. Trump’s unpredictability and transactional approach could pose risks for India.

Two issues in particular are bound to test the relationship, and most likely soon. During the campaign, Mr. Trump criticized India as gaining an unfair advantage in trade by maintaining high tariffs. And India could be swept up in the controversy if Mr. Trump follows through on his promise of mass deportations of illegal immigrants.

Indians make up the third-largest group of illegal immigrants in the United States, according to the Pew Research Center. If Mr. Trump sends large numbers of Indians back to their home country, it could be a major embarrassment for Mr. Modi.

Amita Batra, a New Delhi-based economist and trade expert, said that India should see warning signs in Mr. Trump’s threat of higher tariffs even against America’s traditional allies, as well as his stated willingness to unravel deals with countries like Mexico and Canada that his own first administration had put in place.

“You may say we are on great terms with Trump, we have an easy relation with the United States, but how Trump views that at a particular time is a different question altogether,” Dr. Batra said at an event at the Center for Social and Economic Progress in New Delhi. “India has to be very cautiously approaching Trump 2.0.”

During the interview, Mr. Garcetti described the bilateral relationship as “the most compelling, challenging and consequential” for both countries.

A former Democratic mayor of Los Angeles, Mr. Garcetti arrived in New Delhi in April 2023, after the mission had remained without an ambassador for two years. His confirmation process had hit a wall over accusations that he had overlooked complaints of sexual harassment by an aide when he was mayor.

He made up for the time lost with a burst of energy and outreach like that of a politician in campaign mode.

He was everywhere, from cricket grounds to cafeterias to cultural programs. Sporting a leather jacket, he even got behind the piano to open for the jazz legends Herbie Hancock and Dianne Reeves, who had come to perform at the Piano Man Jazz Club in New Delhi.

But by the time Mr. Garcetti tried his hand at dancing to a viral Bollywood tune at a Diwali celebration, relations between the two countries had hit major obstacles.

In India, right-wing trolls had seized on the U.S. allegations of Indian government involvement in a plot to assassinate an American citizen advocating a separatist cause in India. That, along with the U.S. indictment of Gautam Adani, the business mogul, was evidence that the United States was trying to dampen India’s inevitable rise, the nationalist online voices argued.

The Biden administration appeared intent on addressing the assassination episode quietly with New Delhi, demanding accountability without allowing it to become a major diplomatic sore point.

“On Capitol Hill, within the White House, I think with those in the know it was a real moment of reflection and pause,” Mr. Garcetti said of the assassination case. “It didn’t pause the momentum — you know, relations between countries are always multifaceted and simultaneous and not just between governments. But I think it was an immediate gut check.”

Mr. Garcetti said that the Biden administration had been reassured by India’s response. New Delhi had accepted the U.S. demand, he said, “not just for accountability but for systemic reform and guarantees that this will never happen again.”

An Indian government inquiry that concluded last week recommended legal action against an unnamed person with “earlier criminal links.” It said that the action “must be completed expeditiously,” in what analysts saw as an attempt to begin the Trump era with a clean slate.

“If we want to cooperate in other areas that are important to us, intelligence sharing, et cetera, trust is the basis of everything,” Mr. Garcetti said. “But I’ve been pretty blown away with how trust can deepen through a challenge.”

One question hovering over the deepening ties between the two countries is whether India can truly emerge as an alternative to China in global supply chains — something that Mr. Garcetti also wondered.

India has reaped only a small part of the windfall from the moves away from China, with businesses preferring places like Vietnam, Taiwan and Mexico, where it is easier to set up operations and where tariffs are lower.

Mr. Garcetti said India had made dramatic leaps after opening up its economy only in the 1990s, years after China. He picked up his iPhone to illustrate a widely highlighted recent success: About 15 percent of iPhone manufacturing now happens in India, a figure that could continue growing rapidly, he said.

More broadly, though, India still struggles to attract foreign investment, despite improvements in infrastructure and some streamlining of regulations. Manufacturing is not growing quickly enough to bring India the jobs it desperately needs.

“Where India’s leaving a lot of progress and jobs and growth on the table is figuring out a better way to make it seamless and frictionless to invest here for export,” Mr. Garcetti said. “Because it’s still, you know, for so many components of manufacturing, one of, if not the, highest tariffed economies.”

“They’re not wrong to look and say it used to be 95 percent worse,” Mr. Garcetti said. “But if that 5 percent is still double your competitor or 10 times your competitor — companies, you know, are like water. They flow where gravity takes them.”