KABUL, Afghanistan — Six years after being driven from power, the Taliban are demonstrating a resilience and a ferocity that are raising alarm here, in Washington and in other NATO capitals, and engendering a fresh round of soul-searching over how a relatively ragtag insurgency has managed to keep the world’s most powerful armies at bay. The mounting toll inflicted by the insurgents, including nine American soldiers killed in a single attack last month, has turned Afghanistan into a deadlier battlefield than Iraq and refocused the attention of America’s military commanders and its presidential contenders on the Afghan war.