For months leading into the American elections last fall, the prospect of a second Trump presidency deepened uncertainty among Ukrainians over how enduring American support would prove in a war threatening their national survival.

After President Volodymyr Zelensky’s disastrous meeting with President Trump in the White House on Friday, many Ukrainians were moving toward a conclusion that seemed perfectly clear: Mr. Trump has chosen a side, and it is not Ukraine’s.

In one jaw-dropping meeting, the once unthinkable fear that Ukraine would be forced to engage in a long war against a stronger opponent without U.S. support appeared to move exponentially closer to reality.

“For Ukraine, it is clarifying, though not in a great way,” Phillips O’Brien, an international relations professor at the University of St. Andrews in Scotland, said in an interview. “Ukraine can now only count on European states for the support it needs to fight.”

An immediate result was that Ukrainians, including opposition politicians, were generally supportive of Mr. Zelensky on Saturday for not bending to Mr. Trump despite tremendous pressure.

Maryna Schomak, a civilian whose son’s cancer diagnosis has been complicated by the destruction of Ukraine’s largest children’s cancer hospital by a Russian missile strike, said that Mr. Zelensky had conducted himself with dignity.