As bad as Pakistan's relationship with the United States may appear to be today, it can get far worse.

Over the past 16 years, which included hundreds of deadly US drone strikes, Osama bin Laden's killing on Pakistani soil and accusations that Pakistan helps insurgents who kill Americans, there is one point of no return that the two reluctant allies have never arrived at: Pakistan closing the air routes to Afghanistan.

It's an action that could all but cripple the US-backed military fight against the Taliban.

It could also be tantamount to Pakistan going to war with the United States.

Even if such a step is seen as unlikely by most officials and observers, Pakistan's ability to shape the destiny of America's longest war is a reminder of how much leverage the country maintains at a time Trump is suspending hundreds of millions of dollars in military assistance.

Pakistan's leverage

“There's some suggestion that we have all of the cards in our hands,” said Richard Olson, a former US ambassador to Pakistan.

“But we don't. The leverage is strong on the Pakistan side as well and arguably stronger than our side.”

Trump's re-commitment of US forces to the fight in Afghanistan makes the stakes high for his administration.

The top US diplomat for South Asia, Alice Wells, made a low-key visit to Islamabad this week, suggesting both sides want to prevent a breach in ties.

Pakistan's cooperation is needed not only to reduce violence in its northern neighbour. It's also critical to any hope of a political settlement with the Afghan Taliban after decades of conflict.

Seeking out alternatives

Defense Secretary Jim Mattis has said the US doesn't expect Pakistan to cut off supply routes.

Even so, the US is seeking out alternatives, a senior administration official said, without elaborating on what those routes might be.

The Pentagon wouldn't discuss the issue, citing operational security, other than to say military planners develop “multiple supply chain contingencies” to sustain their mission.

The administration official, who wasn't authorised to comment by name and demanded anonymity, said it would be “very difficult” but not impossible for the US to get military equipment into Afghanistan if the Pakistan route is shut down.

Restrictions limit what types of supplies can flow through the Northern Distribution Network in Central Asia, set up during the Obama administration amid concerns about relying solely on Pakistan.

Pakistan has cut overland access before. When a US airstrike killed 24 Pakistani soldiers at the Afghan-Pakistan frontier in late 2011, months after the US commando raid that killed bin Laden, Pakistan blocked border crossings into Afghanistan.

The decision sunk US-Pakistani relations to a post-9/11 low point.

Supply trucks that trundle across desert into Afghanistan's southern Kandahar province or into Nangarhar via the mountainous Khyber Pass ground to a halt. Hundreds of containers shipped from the US or the Gulf were left stranded in the Pakistani port of Karachi until mid-2012.

For the US, truck and rail costs inflated by about 50 percent, said David Sedney, a former Pentagon official who organized the alternative northern routes. He said deliveries by air cost three times as much or more.

But the saga, resolved through a US apology, also exposed the limits of Pakistan's leverage, Sedney said.

Pakistan's own economy was hurt, notably the military-dominated trucking industry. And the Afghan war effort, which was then supporting more than 70,000 US troops, compared with around 16,000 now, endured.

That was perhaps the result of Pakistan never closing the air corridor into Afghanistan, which US pilots call “the boulevard.”

It's essential for ferrying ammunition and weapons for US and Afghan forces, and waging war. US intelligence flights and combat missions use it when taking off from US bases in the Persian Gulf or from aircraft carriers in the Indian Ocean.

Grounds for war

Since closing Pakistan's airspace would hinder America's ability to defend its forces in Afghanistan, Olson, the former ambassador, said the US might regard such action as a “casus belli,” or grounds for war. Other former US officials echoed that assessment.

“From what I can tell we don't actually have any serious alternative,” said Daniel Markey, a South Asia expert at the John Hopkins School of Advanced International Studies.

Sedney said the Northern Distribution Network, which fell out of use after most US forces were withdrawn from Afghanistan by late 2014, could be restored with astute US diplomacy.

Nations such as Uzbekistan, Kyrgyzstan, Tajikistan and Kazakhstan all have been used before for transporting mostly nonlethal supplies. Poor US relations with Russia could make the task trickier, however. Moscow wields significant influence over these former Soviet states.

Pakistan's options

Pakistan is weighing its options carefully. The suspension of around $1.2 billion in assistance and Trump's accusations of Pakistani “lies and deceit" for allowing Taliban havens have stirred anger and demands from Pakistan Tehreek-i-Insaf Chairman Imran Khan for both land and air links to be cut.

Pakistan's ambassador in Washington, Aizaz Chaudhry, indicated such steps weren't imminent, urging greater US cooperation on counterterrorism.

But he warned that further downward spiralling in US-Pakistani ties could create a situation in which “everything will be on table.”

Chaudhry cited Pakistan's longstanding complaints that its efforts have been unappreciated, claiming that most leaders of the Haqqani network which the US hopes to eradicate have fled to Afghanistan. Critics say Pakistan's military only targets insurgents threatening Pakistan itself.

“The problem is we have a porous open border and it's like a revolving door,” Chaudhry told the Associated Press.

“These elements tend to come back, and travel back and forth, but there is no organised presence or safe havens inside Pakistan.” Republicans and Democrats in America aren't sold.

Lawmakers have urged targeted financial sanctions against Pakistani intelligence officials linked to militants, and for Pakistan to lose its “non-Nato ally” status that offers preferential access to US military technology.

Zalmay Khalilzad, a former US ambassador in Kabul, is among hawks advocating Pakistan be declared a state sponsor of terrorism unless it cooperates.

But others who've worked with the Pakistanis fear coercion could backfire at a time they're hedging their bets, unsure America will win in Afghanistan.

A tacit Pakistani alliance with the Taliban will appear “more important to them than ever as we turn once again from an ally into an adversary,” said Ryan Crocker, who was US ambassador in Pakistan and Afghanistan.