To avoid a direct clash, the boundaries between British India and Afghanistan were demarcated in 1893. The Afghan Amir renounced his claim to certain areas, which were permanently annexed to British India and the successive Afghan rulers recognised it as international frontier. However, on the eve of the partition of India, the Afghan Government demanded that its former territories should be handed back to Afghanistan. Since then it has been a contentious issue between Afghanistan and Pakistan. The purpose of the present research is to unravel and analyse the factors responsible for the demarcation of the Durand Line and the new dimensions of the whole scenario in the wake of the tragic events of 9/11 and the US invasion of Afghanistan in October 2001. Other issues dealt with include: (a) the ouster of the Taliban regime; (b) the regrouping of the Taliban and al Qaida in the tribal areas of Pakistan and their intrusion into Afghanistan; (c) the presence of Pakistani troops in the tribal territories and its repercussions; (d) the unchecked cross-border infiltration and its impact; and (e) the fencing of the Durand Line and the Afghan reaction.