The US-led invasion of Afghanistan in October 2001 resulted in the quick defeat of the Taliban regime. The transitional process, guided by the Bonn Agreement, established a new constitution and successful presidential elections. Hamid Karzai was elected president for a second term in October 2009 despite claims of widespread fraud and lower voter turnout. However, the Taliban-led insurgency continued to cause major insecurity throughout the country and to challenge both the efforts of the NATO-led International Security Assistance Force (ISAF) and the authority of the Afghan government. Following another election marred by fraud claims in 2014, a national unity government was formed with the two leading candidates, Ashraf Ghani and Abdullah Abdullah, respectively made president and chief executive officer. After a ‘surge’ in troops in 2009, NATO handed over security responsibilities to the Afghan National Security Forces (ANSF) at the end of 2014 following a phased withdrawal. Some 10,000 US and NATO troops remain in Afghanistan with a mandate to train, advise and assist; this number is projected to fall to around 1,000 by the end of 2016. The security situation deteriorated severely in 2015, with both civilian and ANSF casualties increasing sharply as the government struggled to maintain control beyond Kabul. The Afghan-owned and led peace process with the Taliban brought about the first official meeting between the two sides, but was again halted with the revelation that Taliban leader Mullah Omar had died in 2013. Mullah Akhtar Mohammad Mansour is the new leader of a resurgent but divided Taliban, the strength of which was demonstrated when it briefly captured a major city, Kunduz, in September 2015 – the first time it had done so since 2001. Meanwhile, a new threat has emerged in the form of militants affiliated with the Islamic State, also known as ISIS or ISIL, who have seized territory in eastern Afghanistan.