The speed of the Taliban advance in Afghanistan appears to have taken many by surprise - regional capitals seem to be falling like dominoes.   The momentum is clearly with the insurgents, while the Afghan government struggles to keep its grip on power. This week, one leaked US intelligence report estimated that Kabul could come under attack within weeks, and the government could collapse within 90 days. So how did it unravel so quickly? The US and its Nato allies - including the UK - have spent the best part of the last 20 years training and equipping the Afghan security forces. Countless American and British generals have claimed to have created a more powerful and capable Afghan army. Promises that today appear pretty empty. Strength of the Taliban The Afghan government should, in theory, still hold the upper hand with a larger force at its disposal. The Afghan security forces number more than 300,000, on paper at least. That includes the Afghan army, Air Force and police. But in reality the country has always struggled to meet its recruitment targets. The Afghan army and police has a troubled history of high casualties, desertions and corruption - with some unscrupulous commanders claiming the salaries of troops who simply didn't exist - so called "ghost soldiers". In its latest report to the US Congress, the Special Inspector General for Afghanistan (SIGAR) expressed "serious concerns about the corrosive effects of corruption... and the questionable accuracy of data on the actual strength of the force". Jack Watling, of the Royal United Services Institute, says even the Afghan army has never been sure of how many troops it actually has. Added to that, he says there have been problems with maintaining equipment and morale. Soldiers are often sent to areas where they have no tribal or family connections. One reason why some may have been so quick to abandon their posts without putting up a fight. The Taliban's strength is even harder to measure. According to the US Combating Terrorism Center at West Point, estimates suggest a core strength of 60,000 fighters. With the addition of other militia groups and supporters, that number could exceed 200,000. But Dr Mike Martin, a Pashto-speaking former British army officer who tracked the history of the conflict in Helmand in his book An Intimate War, warns of the dangers of defining the Taliban as a single monolithic group. Instead he says "the Taliban is closer to a coalition of independent franchise holders loosely - and most probably temporarily - affiliated with one another". He notes the Afghan government, too, is as riven by local factional motivations. Afghanistan's shape changing history illustrates how families, tribes and even government officials have switched sides - often to ensure their own survival.