Testifying before the House Appropriations Defense Subcommittee, the former four-star general said Washington's war policy for Afghanistan and its impact could no longer be limited to within the country's borders.
The initial plan, according to Mattis, is to send additional special operations forces to assist Afghan forces and to counter Taliban support in Pakistan, along with beefing up air power and artillery. That means there are more than 6,000 American troops in Iraq and more than 900 American troops in Syria. He said that while counterterrorism efforts in Afghanistan are making progress in weakening al-Qaida and IS, "their defeat will come about only by giving our men and women on the ground the support and the authorities they need to win".
For years after the initial US -led invasion, former Representative Ron Paul (R-Texas), the leading non-interventionist in Congress, continually noted that no war had been constitutionally authorized within Afghanistan - only the use of force against those responsible for the 9/11 attacks.
This comes after Mattis's testimony to the Senate Armed Services Committee earlier on Tuesday in which he pressed for troop increases to combat a "surging" Taliban.
ABC News looks at the change and whether anything will change in the US military posture in Iraq, Syria and Afghanistan.
Obama set a cap a year ago of 8,400 troops in Afghanistan after slowing the pace of what he hoped would be a US withdrawal.
The US now has around 8,400 soldiers in Afghanistan with about another 5,000 troops from North Atlantic Treaty Organisation allies.
"I believe Jim Mattis will make a great decision on Afghanistan", McChrystal said hours before the announcement. Mattis' statement was made during a tense exchange with the committee's chairman, John McCain (R-Ariz.).
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The finality of the decision isn't entirely clear. "But I don't think that takes away President Trump's responsibility for that decision".
The Pentagon is reviewing the strategy for Afghanistan, which Mattis said may take several weeks. Dick Durbin, D-Ill., said Trump has "no military or political experience whatsoever".
Mattis signaled his intention to brief lawmakers on a new strategy by mid-July.
As such, Mattis is looking to end the war as soon as possible.
"And we will do so", the Pentagon chief said before the House subpanel.
An Afterthought: Let's suppose for a moment that Trump actually wanted to end the Afghan war.
There have been nearly 2,400 US military deaths in Afghanistan since 2001.