The 9/11 terrorist attack on the World Trade Center changed the definition and manner in which the world viewed terrorism. Following the attacks, India offered major support to the US through its counter-terrorism efforts in Afghanistan.
Earlier, whenever India would face terrorist strikes, especially in Jammu and Kashmir, the US would consider it a domestic issue and refuse to acknowledge the tacit role of Pakistan in spreading terrorism.
At the diplomatic level, then Prime Minister Atal Bihari Vajpayee visited the US to reaffirm India's support on counter-terrorism cooperation. The visit paved the way for deeper intelligence sharing between the two countries, especially relating to the Indian Ocean and South Asia.
During the visit, both sides issued a number of policy statements jointly on the scope of counter-terrorism cooperation. The joint statement issued on November 9, 2001 said, "Since September 11, the people of the United States and India have been united as never before in the fight against terrorism... They noted that both countries are targets of terrorism, as seen in the barbaric attacks on September 11 in the United States and on October 1 in Kashmir.
"They agreed that terrorism threatens not only the security of the United States and India, but also our efforts to build freedom, democracy and international security and stability around the world. As leaders of the two largest multicultural democracies, they emphasised that those who equate terrorism with any religion are as wrong as those who invoke religion to commit, support or justify terrorist acts."
Post 9/11, the two sides decided to expand the scope of the India-US Defence Policy Group (DPG) to cover military-to-military cooperation in counter-terrorism.
The DPG decided to supply American sensors for monitoring India's land borders. In 2004, both sides inked the Next Steps in Strategic Partnership, and the following year, in a boost to defence ties, New Delhi and Washington adopted a new framework for the US-India defence relationship, in which counter terrorism was one of the major elements. The 2008 terrorist attack in Mumbai helped intensify their bilateral engagement.
The killing of Osama bin Laden in Abbottabad in 2011 forced the US to seriously rethink its Pakistan policy and led the Barack Obama administration to suspend $800 million in military transfers, including $300 million to reimburse Pakistan for expenses incurred in fighting terrorism.
The rise of the Islamic State also necessitated collective and strong efforts by the countries, including India and the US, in the sphere of counter-terrorism cooperation.