The US-Afghan relationship is about to be battered by its fourth crisis in four months. American troops have this year inadvertently burned Korans, massacred Afghan villagers, and been filmed urinating on Taliban corpses. And now there are more shocking photos of troops cavorting with corpses. This is hardly the basis for trust and friendship.

The relationship between Hamid Karzai and the West has been disintegrating since his fraud-marred re-election in 2009. But Afghanistan's future hinges less on Karzai, who will be gone in 2014, and more on the answers to three questions. The first is how quickly US forces will drawdown from the country. The second is whether the Afghan National Security Forces (ANSF) can replace them. The third is whether the Taliban are open to a political settlement with the Kabul government.

Over the past year, it has become clear that the US-led counterinsurgency strategy in Afghanistan has failed. This is not about the coordinated Taliban attacks of the sort of witnessed over the weekend. Those are dramatic, but have little lasting impact. The real problem is twofold: the failure of the Afghan government to end corruption, and Pakistan's continued support for the insurgency.

In the face of these odds, the French have already given up and announced an early exit. The US has announced the withdrawal of all combat forces by 2014, and a complete handover to Afghan forces next year. Western hopes are now pinned on the Afghan army serving as a bulwark against the return of the Taliban.

However, this strategy has several problems. First, it means that perhaps tens of thousands of Western troops will need to stay in Afghanistan after 2014 to train up their Afghan counterparts. Even if these are special forces rather than regular soldiers, they will still need bases. Not only does that make it harder to reach a deal with the Taliban, who won't accept any US presence, but it also requires what is known as a Status of Forces of Agreement (SOFA) to give US troops immunity.

It was the inability to reach such an agreement that compelled the US to withdraw from Iraq. The US has made concessions to President Karzai – giving him control over night raids against suspected insurgents, and responsibility for a key prison – but a deal still isn't guaranteed.

Second, Afghan forces remain inadequate and unaffordable. Only a tiny fraction of the army is capable of operating without extensive American help, and its present strength (350,000 men) will cost $7-8bn annually. The solution has been to cut that down to 220,000 in five years. But those numbers may not be enough to secure the country. Nor is it clear whether the US Congress will agree to stump up the cash in the interim. European states no longer see Afghan instability as a threat to their vital interests. They have enough financial problems of their own without also throwing good money after bad.

Third, a political settlement to the war looks out of reach. After some major steps earlier this year, such as the opening of a Taliban political office in Qatar, the group suspended talks a month ago. Yet even if negotiations were to resume, the thorniest problem may lie in splits within the Taliban. Naso's own policy of raids has eroded the middle-ranks of the Taliban, and their replacements are both closer to al-Qa'ida and less amenable to compromise. Of course, the war may be lost even if things fall perfectly into place. We may be quibbling over little more than the scale and speed of defeat.