In an unannounced trip to Iraq on Wednesday, President Donald Trump staunchly defended his decision to withdraw USA forces from neighbouring Syria despite a drumbeat of criticism from military officials and allies who don't think the job fighting Islamic State militants there is over.
Mattis and other top advisers had cautioned Trump not to order a precipitous withdrawal from Syria, where roughly 2000 U.S. troops enabled a Kurdish-led militia to roll back the self-declared caliphate of the Islamic State.
First Lady Melania Trump accompanied him on the surprise trip.
The US presence in Syria was not meant to be "open-ended", he said, adding that other wealthy nations should pay for rebuilding Syria.
Talking to reporters, Trump defended his Syria decision and said people were going to eventually understand his reasoning.
While there has been no full-scale violence in Iraq since ISIL suffered a series of defeats past year, some 5,200 U.S. troops train and advise Iraqi forces still waging a campaign against the group.
The terrorist group has lost roughly 99% of its territory in Iraq and Syria and is no longer in control of any major city.
Trump's first visit to US troops deployed overseas came during the ongoing government shut-down in Washington, which began last week after the Trump Administration and Congress failed to agree over funding for a wall at the USA border with Mexico.
"Because you have the right to have a Secretary of Defence whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position", the letter reads. Go ahead, take more time.
US President Donald Trump speaks to members of the US military during an unannounced trip to Al Asad Air Base in Iraq
Trump also said he had no plans to withdraw the 5,200 US forces in Iraq. "We've knocked out about 99% of the caliphate (caliphate is the Islamic State's dream realm)", he had said.
The president told reporters that he wants Iraq to still be used as a base to stage attacks against the Islamic State if needed - and added that the USA can attack ISIS "so fast and so hard" that they 'won't know what the hell happened'.
The U.S. military said Tuesday that coalition warplanes and artillery guns launched airstrikes against ISIS fighters inside Syria between December 16 and 22.
George W. Bush made four trips to Iraq as president and President Barack Obama made one.
"They are very concerned in Iraq about the 2,000 troops pulled out of Syria", he said.
Mr Trump wants $5billion (£3.95bn) in funding for the wall but the Democrats have said no. The U.S. -led military coalition continues a daily battle against the remaining ISIS fighters holed up in a stretch of desert along the Middle Euphrates River Valley, near he Iraq-Syria border.
He cited worries about the safety of the First Lady, and said that's why he made the precautionary decision to arrive at night - to minimize visibility of his plane landing. He said some troops "can now return home to their families".
It comes a week after Mr Trump stunned his national security advisers by announcing that he would withdraw USA troops from neighbouring Syria where they have been fighting Islamic State militants.
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