What did Donald Trump do today?

He gave Russia, Iran, and the Assad regime in Syria a major geopolitical victory.

This morning, Trump announced via tweet a complete withdrawal of U.S. troops involved in the ongoing Syrian conflict. His justification: that the United States had "defeated ISIS in Syria, my only reason for being there."

Whether it's wise to withdraw the roughly 2,000 American servicemembers stationed in Syria is debatable—but the side of that debate that Trump has suddenly put himself on has had very few takers before now. Trump's own Defense and State Departments argued strenuously against it, with one military official going so far as to suggest that Trump might simply be trying to offer a distraction from his criminal and political struggles at home.

But Trump's supposed reason—the defeat of the Islamic State in Syria—is fictional. The territory that ISIS directly controls is reduced lately (because it has been taken over by the Assad regime, which the United States also opposes) but the faction itself is still present and dangerous.

By declaring ISIS defeated before it actually is, Trump is, in effect, quoting Iranian propaganda. It's not clear whether he knows or understands this.

The simplest measure of the geopolitical implications of Trump's declaration today is the reaction it provoked in others. Russia, which is allied with the genocidal Assad regime, was ecstatic; Republicans and Democrats alike were horrified, and the Pentagon was caught so badly off-guard by the sudden shift that it was unable to offer any explanation. 

Why should I care about this?

• Presidents should make major geopolitical decisions based on the advice of military and diplomatic experts, not political expediency.

• It's bad if a president suddenly does something that his whole government opposes but which helps the hostile foreign power that helped get him elected.

• There are no simple solutions to problems like Syria.