The United States has no coherent foreign policy under President Donald Trump and is being guided solely by the president's often contradictory tweets, says historian and retired Army Colonel Andrew Bacevich.

"To have a foreign policy, you have to have a worldview. You have to have a set of principles. You have to have an agreed upon definition of what your interests are, a hierarchy of interests … I don’t think that President Trump himself has the capability to come to those sorts of judgments," Bacevich told Greater Boston Wednesday.

"What we have is [we] make it up as we go along. What we have is the president tweeting and therefore contradicting what his subordinates may have said yesterday or the day before," Bacevich continued.

Bacevich is the president of the Quincy Institute for Responsible Statecraft and author of the forthcoming book, "The Age of Illusions: How America Squandered Its Cold War Victory."

Bacevich also told host Jim Braude that he believes it is time for the United States to leave Afghanistan. Last week it appeared the United States was nearing a peace deal in Afghanistan when Trump abruptly canceled a summit he said he had planned at Camp David for the Afghan government and Taliban leaders. Trump later declared the peace process "dead."

"It does seem to me that at the end of the day, we have to leave it to the Afghans to determine their future — and they will do so. It would be great if they could do so that would result in some kind of reconciliation between the Taliban and the rest of the population," he said. "But If we stay all we are doing is prolonging a war that…is unwinnable."