When America launched its combat mission in Afghanistan, now its longest ever war, swift victories for its special forces, smart bombs and all-seeing drones briefly made progress seem assured.

Now, more than 12 years later, as Washington attempts to withdraw from the battlefield, it finds itself in a political morass as complex as any mountain skirmish.

Their nominal ally, the unpredictable Afghan president Hamid Karzai, is refusing to sign a security deal to cover the presence of US forces beyond the end of 2014, when the NATO mission ends.

Karzai owes his power to the US-led intervention to overthrow the former Taliban regime, but the Islamist militia was never fully defeated and fighting will continue with or without American troops.

Why is the United States frustrated?

US officials believed they had an agreement after months of negotiations and an endorsement from a council of Afghan elders convened by Karzai, but patience is now wearing thin.

The Afghan leader's foot-dragging has complicated efforts by the US military and intelligence agencies to plan a possible post-2014 mission and the troop drawdown already underway.

"You can't just keep deferring and deferring because at some point the realities of planning and budgeting collide," Defense Secretary Chuck Hagel told reporters last week.

At base, the US option on the table are either to pull out all their troops and leave Afghan forces to fight on alone, or to leave roughly 10,000 in place to train and support government forces.

The Chairman of the Joint Chiefs of Staff, General Martin Dempsey, has said the Pentagon has at least until the "early summer" before a delay would start to restrict his military options.

What is the impact on NATO?

NATO members and allies considering deploying troops after 2014 have been waiting on the US-Afghan pact to negotiate their own legal arrangements with Kabul for their forces.

With no Bilateral Security Agreement (BSA) in place, they will find it hard to win political backing from their own parliaments.

So, given the already weak public support for the mission, Karzai's refusal threatens to erode the coalition.

NATO Secretary-General Anders Fogh Rasmussen said Kabul "should not underestimate the negative impact" Karzai's refusal to sign has had on the 44 countries with troops in Afghanistan.

What is the fallout for international aid?

Karzai's stalling has already triggered one concrete result: the US Congress has cut development aid to Afghanistan in half for 2014, down to $1.12 billion.

Senator Lindsay Graham, an outspoken supporter of the US military presence, has become so frustrated with Karzai that he has warned of possibly cutting off all aid.

"This idea of trying to squeeze more out of us has got to stop," the Republican said.

The Afghan government relies heavily on international aid, and without it the country's economy would essentially collapse.

The cost of funding 352,000-strong security forces in 2013 came to $6.5 billion alone.

But after more than a decade of pouring billions into Kabul, enthusiasm among donor countries has begun to wane.

Why are Afghan forces nervous?

The Afghan police and army have improved, but the possibility that no US-led force will remain after this year has made them anxious, according to General Dempsey.

The Afghan troops, he said, are worried "about whether we're going to be there to support them."

In the country's east, families who used to send at least one of their sons to serve in the national army to earn a regular paycheck now hesitate, according to a report by the US Institute of Peace.

How does it affect the economy?

Uncertainty about the BSA has had a ripple effect across the Afghan economy.

Land and property prices are falling, investors are more cautious and construction has slowed, according to the USIP study.

Afghans spend less and save more, hedging their bets in case they have to flee the country if security deteriorates. Business owners are importing fewer goods.

Farmers are also planting more opium poppy to cushion themselves from any potential shocks and others are keeping opium seeds on hand as insurance.