Now James Mattis leaves Trump increasingly isolated in the White House, we will see his real agenda unleashed in 2019

Trump is sure to double down on the things that his supporters look for – dealing harshly with nations like Iran, hardline immigration policy and rolling back environmental laws

The resignation of James Mattis as US defence secretary has sent a chill down the spines of many in Washington, and likely several leaders of America's allies around the world.

Mattis was the last of the so called “axis of adults” that were supposed to apply the brakes when Donald Trump’s impulsive tendencies got the better of him. Last year, the chairman of the Senate Foreign Relations Committee, Republican Bob Corker, said he was thankful that secretary of state Rex Tillerson, Mattis and White House chief of staff John Kelly were around to help guide the president.

First Tillerson was forced out, while Kelly and Mattis are now serving their last days in the administration all for the same reasons – they can no longer work for the president, or he can no longer work with them.

Much of the fear surrounding the resignation of Mattis, and that is what many in congress are describing it as, relates to his decades of military service. He has seen much more of the world than Trump, a novice in all areas of policy before entering the White House, could ever see. In a nation that prides itself on its respect for the military, when someone of such long-standing feels the need to quit people take notice.

Mattis’s exit leaves the White House increasingly isolated. It is one of a number of recent incidents that has raised the hackles of even Republicans in congress – from how the president responded to the killing of Saudi journalist Jamal Khashoggi to how he has flip-flopped on border wall funding.

But being isolated from outside voices is seemingly what brings out the real Trump. Surrounding himself with advisors that think like he does has been a feature of his ever-changing White House.

Trump believes that what gives him power is setting himself at odds with everyone he can. The removal of influences like Mattis – as well as Democrats taking control of the House of Representatives – will only accelerate this trend.

Stripped of dissenters around him, the president can power through with his America First agenda in 2019. The split congress will likely dilute the impact of his agenda, but that won’t stop the president from trying.

Appearing to agree to a spending bill to keep the government open and then suddenly backtracking after seeing the negative reaction among his biggest supporters to him giving up on border wall funding shows that he really only cares about one thing – how he looks to his base.

If the checks and balances aren’t there to put a halt to some of the president’s whims, we will see much more of policy by tweet, impromptu press conferences and reaction to Fox News. He is sure to double down on the things that his supporters look for – dealing harshly with nations such as Iran, hardline immigration policy and rolling back environmental laws.

Many are rightly worried about the foreign policy implications of Mattis leaving the White House – the US pulling out of theatres of conflict without warning or the denigrating of international alliances – but home policy could be equally affected.

Without the stabilising influence of people like Mattis, we could get see Trump fully unleashed in 2019.