"Make no mistake, this is combat duty, but the Afghan forces will remain in the lead for the fighting", Mattis said at a Senate Armed Services Committee hearing on Afghanistan strategy that took place 16 years after USA troops went into Afghanistan in October 2001.
Replying to a question on what options does the United States have to ensure Pakistan cooperates in fighting terrorism, Mattis said, "Right now, with the growing consensus against terrorism, they will find themselves diplomatically isolated, they will find themselves economically isolated and in increasing trouble as countries that are damaged by this terrorism coming out of there say enough is enough and take steps". "It has much to lose by continuing to harbour terrorists".
Frustration has been mounting in the United States with Pakistan's dodgy record on counter-terrorism and its use of groups sheltering on its soil as a tool of its foreign policy.
Reuters first reported that possible Trump administration responses being discussed include expanding US drone strikes and perhaps eventually downgrading Pakistan's status as a major non-NATO ally. We will firmly address Pakistan's role.
Trump during the presidential campaign railed against the so-called Joint Comprehensive Plan of Action - which was negotiated by the Obama administration along with Russia, China, France, Britain and Germany - and vowed to rip it up.
"It is time for the Taliban to recognize they cannot kill their way to power, nor can they provide refuge or support to transnational terrorists who intend to do us harm", Mattis said. "I repeat, this is totally unacceptable", Sen.
The strong statements on ISI and Pakistan from top officials of the Trump Administration came hours before the Pakistani Foreign Minister Khawaja Asif landed in Washington on a three-day visit.
Tuesday's public calling out of ISI's terrorist ties was the harshest indictment of Pakistan in years.
Mattis said he would give the committee numbers in private sessions. Encouraging & enlightening, " exulted Amrullah Saleh, a former Afghan intelligence chief who has long called out Pakistan's nurturing of terror groups.
Mattis noted that India has a role to play as Pakistan's neighbour and can potentially provide very strong economic benefits to Pakistan if Islamabad ceases to have safe havens for terrorists and figures out a way to carry out its global responsibilities.
Just this past week, the US Senate passed the defence budget that made some payments to Pakistan conditional on it acting against the Lashkar-e-Taiba, which targets India, and the Haqqani Network, which is active in Afghanistan.
But rather than heed the warning, Pakistan ramped up its histrionics, canceled the scheduled visit of two senior American officials to Islamabad, and demanded a high-level engagement with the U.S. to discuss the matter.
Attending the annual UN General Assembly sessions in NY last week, Asif had slammed United States for being responsible for the rise of terrorist groups in Pakistan.
Allegations against ISI and Pakistan was raised by United States hours before Pakistani Foreign Minister Khawaja Asif landed in Washington for a three-day visit. The officials spoke on condition of anonymity because they were not authorized to publicly discuss planning for the event.
Last month, the Pentagon, after long insisting that 8,400 troops were deployed in Afghanistan, admitted that it actually has about 11,000 troops there.
The rules of engagement have also changed, Mattis said.