Editor's note: Elizabeth A. Cobbs is the Melbern Glasscock Chair in American history at Texas A&M University and a senior fellow at Stanford University's Hoover Institution. Kyle Longley is the Snell Family Dean's distinguished professor of history and political science at Arizona State University. Kenneth Osgood is a professor of history at the Colorado School of Mines. Jeremi Suri is the Mack Brown Distinguished Chair for Leadership in Global Affairs at the University of Texas at Austin. The views expressed here are theirs. Read more opinion on CNN.

(CNN) We are historians who have researched every president since George Washington, and we've never seen anything like this.

When Donald Trump got on the phone with the president of Ukraine, he had a "favor" to ask. It's not the first time he's reached out to a world leader for personal gain and he has made it clear he sees nothing at all wrong with it.

In fact, several transcripts of similar conversations have reached the public domain , including others from 2017 and some to which the White House sought to limit access. They reveal a striking pattern of a president who consistently uses the Oval Office to advance his explicit self-interest seemingly without regard to national interest.

It is rare to get such a real-time look at presidential conversations with foreign leaders. As historians of US foreign relations, collectively we have read many thousands of similar documents from past presidents. We have also listened to audio tapes of conversations between presidents and their international counterparts. In our numerous books on presidents from George Washington to Barack Obama, we have examined how American leaders conduct US foreign policy — the good, bad, and ugly. Nothing really surprises us anymore.

Until now.

Trump's documentary record differs dramatically from his predecessors. A worrisome thread runs through each conversation. Trump appears laser-focused on his own fortunes to the exclusion of the national security of the United States. Unfortunately, this is part of a larger and startling pattern of Trump promoting his personal agenda ahead of the nation's interests.

Many examples exist. When speaking with Ukraine President Volodymyr Zelensky in July, Trump concentrated on soliciting help to discredit a political opponent, former Vice President Joe Biden. Missing was any discussion of US national interests. Trump never mentioned the shared US-Ukrainian goal of containing Russia's ambitions, a cornerstone of the relationship. Instead, he pushed a personal agenda — and Zelensky responded by bragging about staying at a Trump property, to the financial benefit of America's president.

Just days before, Trump had halted US military aid to Ukraine , apparently without a policy review and to the worry of members of Congress. Zelensky has said publicly that he feels no pressure to investigate the Bidens. But the halting of aid surely sent a message to the Ukrainians that they must do even more to please the president — and recklessly endangered the security of Ukraine, Western Europe and indeed our own country.

In the history of American foreign relations, we are unaware of any prior case — in 230 years — of a president asking a foreign leader to intervene in American domestic politics.

In 2017, he asked Mexican president Enrique Peña Nieto to help get him out of a " political bind ." Mexico's continued refusal to pay for a border wall contradicted a core campaign promise. Trump told Peña Nieto that little mattered to him other than saving face. He admitted that Mexico's silence on the wall was not "important" for national security, but "politically this might be the most important."

That same year, Trump's narcissism dominated a telephone call with Australian Prime Minister, Malcolm Turnbull . The conversation turned to a review of 1,250 migrants fleeing war zones. Since Australia had a standing policy of not accepting anyone who arrived unauthorized by boat, the United States under President Obama had previously agreed to consider a swap of Central Americans seeking refugee status in the US with those seeking refugee status in Australia. Trump explained his intent to negate his predecessor's promise because "it is embarrassing to me." The US president framed it not as a policy choice, but as something that would make him "look so foolish," "like a dope," and "kill me" politically. A joint foreign policy challenge with a close ally became an ego trip for the president.

When Turnbull tried to turn the conversation to urgent matters of grave concern, namely the war in Syria and the nuclear ambitions of North Korea, Trump was not interested. He squeezed in one last complaint about how the refugee deal was "an embarrassment to me." Then he hung up.

To be sure, Trump is hardly the first president to fret about how international problems affect how he is perceived at home. Presidents always keep one eye on domestic politics when making foreign policy decisions. But Trump's actions are different. In these conversations there is little discussion of the national interest. His conversations are about him, his image and his needs. He turns policy meetings into short-term political deals with no thought about long-term consequences. He repeatedly stresses how much he values reciprocity but frames it in terms of what he gets personally. No other president has acted with so little evident regard for the nation's core interests.

Finally, Trump repeatedly gives foreign leaders incriminating evidence they can use against him. In asking Zelensky to assist in undermining a likely campaign opponent, Trump made what his advisers recognized as an unconstitutional request. The mere fact gave Zelensky leverage over Trump, should he choose to use it. In much the same way, the Trump campaign's outreach to Russia in 2016 provided Vladimir Putin with compromising material to hold over the future president of the United States. Trump's personalization of diplomacy weakens him and undermines our security.

The revelations of this past week, and the patterns manifest in Trump's own words over the past three years, suggest we cannot trust that experienced senior advisers will tame this presidency, as many had hoped. We must evaluate the Zelensky transcript and the whistleblower report alongside previous evidence that our president is unable and unwilling to place the nation's interests above his own.

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No one knows if Congress will decide that these actions rise to the level of impeachable "high crimes and misdemeanors." We do know history will judge them harshly, because Donald Trump is not fulfilling the most solemn duty of every president: to protect the nation's security.