In the aftermath of Lisbon Summit of NATO in November 2010, US, NATO and Afghanistan agreed to create conditions for “irreversible transition to full Afghan security responsibility and leadership” by the end of 2014. This requires internally an increased pace in the strengthening of administrative judicial, political and security institutions in Afghanistan. Externally, to manage the post-transition responsibilities a supportive role by the neighbouring countries is essential. In this backdrop it is crucial to assess the nature and dynamic of internal and external security challenges to Afghanistan. The study provides useful insights into the nature of complexity that characterizes the behavior of regional actors. It examines the security implications of US exit strategy not only for Afghanistan but in the region and beyond.