One called President Trump’s decision “an egregious act of betrayal.” Another said the policy could be “the biggest mistake of his presidency.” A third said Mr. Trump “is in danger of losing the mandate of heaven.”

Conservative Christians have ardently stood by Mr. Trump at most every turn, from allegations of sexual misconduct to his policy of separating migrant families at the border and the Russia investigation.

But this week, some of Mr. Trump’s top evangelical supporters broke rank to raise alarms over his move to withdraw troops from Syria, which prompted Turkish forces to launch a ground and air assault against a Kurdish-led militia that has been a crucial ally in the American fight against ISIS.

As Turkish warplanes began to bomb Syrian towns on Wednesday, the prominent evangelist Franklin Graham called for Mr. Trump to reconsider his decision, and worried that the Kurds — and the Christian minorities in the region they have defended — could be annihilated.