WASHINGTON — As a conflict between India and Pakistan escalated, Vice President JD Vance told Fox News on Thursday that it was “fundamentally none of our business.” The United States could counsel both sides to back away, he suggested, but this was not America’s fight.

Yet within 24 hours, Vance and Secretary of State Marco Rubio, in his first week in the dual role of national security adviser and State Department leader, found themselves plunged into the details. The reason was the same one that prompted then-President Bill Clinton in 1999 to deal with another conflict between the two longtime enemies: fear that it might quickly go nuclear.

What drove Vance and Rubio into action was evidence that Pakistan’s and India’s air forces had begun to engage in serious dogfights and that Pakistan had sent 300 to 400 drones into Indian territory to probe its air defenses. But the most significant causes for concern came late Friday, when blasts hit the Nur Khan air base in Rawalpindi, Pakistan, the garrison city near Islamabad.

The base is a key installation, one of the central transport hubs for Pakistan’s military. But it is also a short distance from the headquarters of the Strategic Plans Division, which oversees and protects the country’s nuclear arsenal.

The fighting broke out between India and Pakistan after 26 people, mostly Hindu tourists, were killed in a terrorist attack April 22 in Kashmir, a border region claimed by both nations. On Saturday morning, President Donald Trump announced the countries had agreed to a ceasefire.

One former U.S. official long familiar with Pakistan’s nuclear program noted Saturday that Pakistan’s deepest fear is of its nuclear command authority being decapitated. The missile strike on Nur Khan could have been interpreted as a warning that India could do just that.

It is unclear whether there was U.S. intelligence pointing to a nuclear escalation of the conflict. At least in public, the only piece of obvious nuclear signaling came from Pakistan. Local media reported that Prime Minister Shehbaz Sharif had summoned a meeting of the National Command Authority — the small group that makes decisions about how and when to make use of nuclear weapons. Pakistan’s defense minister, Khawaja Muhammad Asif, denied that the group ever met.

But the meeting was being discussed at the Pentagon, and by Friday morning, the White House had clearly made the determination that a few public statements and some calls to officials in Islamabad and Delhi were not sufficient. Interventions by Saudi Arabia and the United Arab Emirates had little effect.

The Trump administration was also concerned that messages to de-escalate were not reaching top officials on either side.

So U.S. officials decided that Vance, who had returned a couple of weeks earlier from a trip to India, should call Prime Minister Narendra Modi directly.

By the U.S. account, Vance pressed Modi to consider alternatives to continued strikes. Modi listened but did not commit to any of the ideas.

The State Department said Rubio talked with Pakistani army chief, Gen. Syed Asim Munir, a conversation made easier by his new role as national security adviser. Over the past quarter-century, the White House has often served, if quietly, as a direct channel to the Pakistani army, the country’s most powerful institution.

Rubio also called Pakistan’s foreign minister, Ishaq Dar, and India’s nationalistic external affairs minister, S. Jaishankar, whom he had met Jan. 22 in Washington.

It is not clear how persuasive he was, at least initially.

But the constant stream of calls from Friday evening into early Saturday appeared to lay a foundation for the ceasefire.

A senior Pakistani intelligence official credited the involvement of the Americans over those 48 hours, and in particular Rubio’s intervention, for sealing the accord.