From making so-called side deals with Ukraine to pulling U.S. forces from northeastern Syria, U.S. President Donald Trump has gone his own way when it comes to conducting U.S. foreign policy. But the Syria decision has sparked widespread opposition in Washington and in the case of Ukraine, critics say Trump sidestepped career U.S. diplomats to further his own interests against a potential election rival. 

Despite criticism, U.S. President Donald Trump is standing by his decision to move U.S. troops from the Syrian-Turkish border, where they fought alongside longtime Kurdish allies.
 
“They stayed for almost 10 years. Let someone else fight over this long, bloodstained sand," Trump said.

This, as the top U.S. official to Syria appeared to distance himself from Trump's decision.
 
“Were you consulted about the withdrawal of troops as was recently done?" Senator Bob Menendez, a member of the Senate Foreign Relations Committee asked James Jeffrey, the U.S. Special Representative for Syria.

“I personally was not consulted," Jeffrey said.

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Elsewhere on Capitol Hill, other diplomats testified about the Trump administration's delay providing approved U.S military assistance to Ukraine.  

A written statement by one described how the White House bypassed normal diplomatic channels to press Ukraine to investigate Democrats and the Bidens in return for military aid.    

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Democratic lawmakers have denounced this.

"The idea that vital military assistance would be withheld for such a patently political reason, for the reason of serving the president's re-election campaign is a phenomenal breach of the president's duty to defend our national security," said Democrat Adam Schiff, Chairman of the House Intelligence Committee.

Trump's actions on Ukraine could threaten his presidency as the impeachment inquiry continues.  
 
“We have agencies, we have tasked areas along government, whose job it is to investigate those. And if President Trump wanted the Bidens investigated for their activities there, it should have been done through the diplomatic channels," said Shannon Bow O'Brien of the University of Texas at Austin.

Those normal diplomatic channels have set the United States apart from countries whose policies are set by autocratic leaders, says terrorism expert Mike Newton, a professor of the practice of law at Vanderbilt Law School.  
 
“The checks and balances, where agencies push back against each other, and really experienced, smart policy makers wrestle with choices and consequences and diplomatic fallout and maybe military best practices," Newton said.

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When this process is bypassed, experts say, U.S. credibility around the world is damaged.
 
“If there are doubts about the President's decisions which there are in the State Department and the Pentagon, and the intelligence community as well as in the Congress, there's real questions about cohesiveness of U.S. foreign policy and whether we have a real strategy," said Mark Simakovsky of the Atlantic Council, an international affairs think tank.
 
Donald Trump was elected on promises to break with traditional U.S. foreign policy, and his supporters show no sign of abandoning him.

But the impeachment inquiry and outcry over his Syria policy show there are limits to his approach.