We now know – not that there was ever much doubt – that Donald Trump writes presidential letters like he talks – with a blustery mix of flattery and threats. His letter to the Turkish president, Recep Tayyip Erdoğan, has all the charm and elegance of an eviction notice from a slumlord, but on White House stationery.

Those who have observed him the longest say this is how he has always expressed himself. The most remarkable aspect of the Erdoğan letter is arguably that it shows the extent to which the distinctions between Trump’s personality and the remaining formal trappings of the presidency have crumbled away.

In a normal White House there is a phalanx of advisers and aides to turn presidential impulse into statesmanlike prose. They have all resigned, been fired or cowed by Trump, who has become ever more convinced of his own “great and unmatched wisdom” with each new foreign policy fiasco. The presidency now truly is Trump unplugged and unleashed.

The tone of the letter is one the former real estate developer might once have used on a truculent tenant who had refused to pay rent until the plumbing was fixed. “Don’t be a tough guy. Don’t be a fool. I will call you later,” Trump writes.

The letter went down as well as could be expected with the autocratic and famously prickly leader of a regional power about to launch an invasion of a neighbouring state. Erdoğan tossed it straight in the bin, according to his aides.

Turkish museum curators will be hoping that the same aides manage to fish it out later. It is compelling documentary evidence of US decline.

Trump writes likes a man wielding overwhelming power, held back only by generous restraint. “I don’t want to be responsible for destroying the Turkish economy – and I will,” he warns. Erdoğan ignored the threats, rightly as it turned out. The US administration’s sanctions – when they came on Monday – were largely hollow: the suspension of a $1bn (£700m) trade deal that never really existed and the doubling of tariffs on Turkish steel sales to the US, which amount to a fraction of a percent of the country’s exports. When his bluff was called, Trump did not even rescind his invitation to the White House for next month.

Erdoğan went ahead with his offensive, though the claim by his officials that the letter was the last straw that goaded him into attack is far fetched. The military operation was well prepared. The last hurdle was the removal of US soldiers from the border, with which Trump obliged three days before sending the letter, after a Sunday telephone call with the Turkish president.

“It’s not because of the letter that Turkey went ahead with the operation,” Sinan Ulgen, a veteran Turkish diplomat and now visiting scholar at the Carnegie Endowment for International Peace, said. “It’s just firstly because of the phone call and then the White House statement – which came before the letter – that Trump was pulling back the troops … So once Turkey got the signal from the White House, that Trump was moving back the troops, that basically gave the green light to the operation.”

The Turks were not alone in seeing the presidential missive as an empty threat in the wake of the telephone call. Bob Menendez, ranking Democrat on the Senate foreign relations committee, told NBC News: “I consider the letter an effort to cover his backside after a firestorm of opposition. I didn’t think it was a serious letter, at the end of the day.”

The Senate on Thursday introduced its own bipartisan sanctions package on Turkey, including restrictions on US arms sales, daring the president to use his veto. If Trump foregoes his veto, it will be a significant defeat. If he uses it, he risks it being overridden by an emboldened Congress, and further underlining the flimsiness of his threats to Erdoğan.

The letter may still end up at the US National Archives alongside the writings of Washington, Lincoln and Roosevelt. The comparison will not be flattering.

Timothy Naftali, a presidential historian at New York University, said: “Since the emergence of the modern presidency, presidents have had a legion of ghost writers and communication specialists and people whose job it was to present the American presidency as a sophisticated and smooth, and at times eloquent operation.

“Due to his inability to accept advice or counselling, Donald Trump has systematically eliminated many of the safeguards that his predecessors have put in place to ensure that however flawed the man, or some day the woman, at the centre of it all, the American presidency will continue to look professional. The Erdoğan letter is the product of an amateur, that should never be an American state document.”