Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of "United States of Jihad: Investigating America's Homegrown Terrorists." The opinions expressed in this commentary are his own. View more opinion articles at CNN.

(CNN)The suicide attack that killed four Americans, including two US soldiers, on Wednesday in Manbij, Syria, is a vivid reminder of the confused and confusing state of US policy in Syria, a confusion that has been engendered almost entirely by President Donald Trump.

We don't know the circumstances of the attack in detail, but it comes at an especially chaotic time.

In a phone conversation on December 14 with Turkish President Recep Tayyip Erdogan, Trump promised Erdogan that all American troops would leave Syria soon since ISIS, in Trump's view, was already defeated. Trump told Erdogan, "It's all yours. We are done."

Five days later Trump posted a video to Twitter in which he said that the approximately 2,000 US troops that are in Syria "are all coming back now."

What was especially odd about this assertion was that Trump had repeatedly warned during his campaign that he wouldn't give America's enemies any heads up about his military plans, as the Obama administration had done when it had announced withdrawal dates for US forces from Afghanistan.

Trump's abrupt shift in Syria policy precipitated the resignation of both the US Defense Secretary James Mattis and Brett McGurk -- who coordinated the many dozens of countries in the global coalition against ISIS at the US State Department.

The Pentagon then let it be known the withdrawal of US troops would in fact not happen immediately, but it would take place over about four months.

Trump's National Security Adviser John Bolton was dispatched to the Middle East to provide some kind of ex post facto cleanup for Trump's hasty decision on Syria, which hadn't been coordinated with the Pentagon or other parts of the US government or American allies such as Israel.

On January 6 in Israel Bolton muddied the waters further, saying that the US would only withdraw from Syria if ISIS was destroyed and the safety of America's Kurdish allies fighting ISIS was guaranteed. Turkey regards armed Kurdish groups in Syria as "terrorists" and without an American security umbrella would likely attack them.

This produced a furious response from Erdogan who thought he had made a deal with Trump. Erdogan said on live TV, "Bolton's remarks in Israel are not acceptable. It is not possible for me to swallow this. Bolton made a serious mistake."

Today US policy in Syria is as clear as mud: Already the Pentagon is withdrawing some military equipment from Syria.

At the same time, Trump has been warned by his own commanders that ISIS has not been entirely defeated in Syria.

Will US forces stay in Syria to finish off ISIS? Clearly the terror group has some life left given the attack in Manbij on Wednesday, for which it claimed responsibility.

Will the US secure some kind of guarantee from the Turks that they won't attack its Kurdish allies in Syria?

None of this is clear and for that we mostly have the impulsive President of the United States to blame, a leader who makes hasty policy decisions based on his gut and without consulting even his own National Security Council.