Washington - Brick by brick, the demolition job has begun: since taking office less than a year ago, Donald Trump has launched an all-out assault on the legacy of Barack Obama.Climate, free trade, health care, immigration, foreign policy - the 45th US president has set about undoing just about everything done by the 44th.All new presidents, of course, break with their predecessor once in the Oval Office, especially if they come from a rival political party.But what is striking is how systematic the hammer blows to Obama's legacy have been.And rather than throw his weight behind new policies or projects, Trump has shown a wilful desire to unpick, shred and erase everything his predecessor accomplished.It's worth noting that each time he buries one of the reforms of the man who sat before him at the "Resolute desk," Trump sounds more like a candidate than a president.Ridiculous trade dealsThe Trans-Pacific Partnership? Within days of taking office, Trump signed an order pulling America out of the free trade accord, the fruit of eight years of negotiations between 12 Asia-Pacific countries, from Chile to Canada and Japan."We're going to stop the ridiculous trade deals that have taken everybody out of our country and taken companies out of our country, and it's going to be reversed," Trump said.Paradoxically, in signing off on the project's demise, Trump was aligning himself more with the left wing of the Democratic party than with the Republican mainstream.The Paris climate accord? Obama played a leading role in attaining that milestone in the effort to combat global warming.Trump pulled out of the agreement signed by 195 countries, claiming that it "punishes the United States" and declaring: "I was elected to represent the citizens of Pittsburgh, not Paris."What about Obamacare, the signature legislative achievement of Obama's first term? After trying in vain to get Congress to repeal it, Trump is now working to bring about its collapse through the regulatory process.And the Iranian nuclear accord? The bid to prevent Tehran from developing a nuclear weapon in return for a lifting of sanctions more than any other bore came to represent Obama's approach to world affairs."This deal will have my name on it," the Democratic president said shortly before it was concluded. "Nobody has a bigger personal stake in making sure that it delivers on its promise."While Trump has stopped short of tearing up the Iran deal, as he threatened on the campaign trail, on Friday he warned he could do so "at any time," raising doubts about the fate of an accord born of years of painstaking diplomacy.A break at any price How to explain the fixation on destroying Obama's legacy at all cost?Trump has held high his determination to fulfil his campaign promises, and give form to a simple slogan: "America First."And his team recalls, with reason, that Obama acted by decree many times when thwarted by Congress. What has been decided by the stroke of a pen can be undone by the stroke of a pen.Historian Jeffrey Engel, however, sees no equivalent in recent decades to Trump's systematic application of the simple principle that "if the other guy liked it, it must be bad."To Engel, the explanation is that Trump's electoral base "never accepted fully Barack Obama as their president.""There was a move among Obama's opponents to delegitimise him and to say that this man is not really president and consequently anything that he did, Trump's base is ready to get rid of," said Engel, who heads Southern Methodist University's centre for presidential history in Dallas, Texas.A notable fact: Obama has until now remained largely silent as his legacy is demolished.American tradition, which is generally respected, holds that a former president should remain above the fray.But, in thinking about his place in history, Obama is also playing the patience card."I think that Obama understands that his legacy ultimately will be defined by how America reacts to Trump in the long term and how Trump's successors act," said Engel.On November 7, 2016, on the eve of the US elections, Obama warned voters "it all goes out the window" if they were to send Trump to the White House.That attempt to rally Democratic voters now seems prophetic.