A chorus of criticism inevitably greeted Mr. Trump’s recent statement that Iranian forces “can do what they want” in Syria. Yet read as a statement of fact rather than the extension of a green light, he stumbled upon a self-evident truth: Notwithstanding Israel’s successful efforts to limit Iran’s importation of advanced weaponry into Syria, Tehran’s position in the country is essentially secure.

Mr. Trump is correct that the better course is to extricate ourselves from Syria, but his fatal error has been in its implementation. Most egregiously, his snap decision during a telephone call with President Recep Tayyip Erdogan of Turkey betrayed our Kurdish partners who led the fight against the Islamic State and risked being exposed to assault by Turkey and the Syrian regime.

A more responsible course would have been to use the prospect of an eventual American withdrawal to help avert a subsequent conflict between the Kurds and their adversaries. If, as he is now suggesting, the president withdraws the troops more gradually rather than right away, that opportunity might still exist.

It requires Mr. Trump to use that time wisely. He should start by abandoning the dangerous notion of Turkey seizing areas controlled by Kurdish forces and instead allow the Kurds — in the absence of long-term American protection — to negotiate an understanding with the Syrian regime. This might entail returning some aspects of the Syrian state to northeast Syria, Kurdish forces retaining their military capacity but lowering its profile and a degree of self-governance for the region.

In Afghanistan too, Mr. Trump’s bottom line was correct. After more than 17 years of combat and a virtual stalemate, at best, for more than a decade, there is little rationale for continuing to expend American blood and treasure on a conflict trending badly, with unclear objectives.

But here too, the self-proclaimed “great negotiator” erred in folding America’s limited cards for nothing in return, rather than using his willingness to withdraw as an incentive for the Taliban — currently locked in negotiations with Mr. Trump’s own diplomats — to make peace.

So much is objectionable about the Trump era that it is hard for critics to know which targets to strike. But principled opposition requires that progressive opponents of President Trump not distort their beliefs for quick rhetorical wins. Whatever administration eventually follows will have many messes to clean up and will need to distinguish those that truly matter.