Secretary of State Mike Pompeo said in a statement a few days after Mr. Hifter’s militia began its attack that “the administration at the highest levels” had made clear that “we oppose the military offensive” and “urge the immediate halt to these military operations.” Most Western governments and the United Nations have also condemned the attack and demanded a retreat.

Mr. Trump, however, told Mr. Hifter almost the opposite, the White House said Friday…

Analysts said Mr. Trump’s endorsement would embolden Mr. Hifter and hamper United Nations efforts to call for a cease-fire. It could also increase the likelihood that his regional sponsors like Egypt or the United Arab Emirates might intervene on his behalf, as each has in the past in Libya.

The policy reversal came as a surprise in part because Mr. Hifter’s forces also appear to be losing ground. His promises of a quick victory have proved false, and his forces appear outmaneuvered by those aligned against them. Most analysts say that he has little hope of exerting his authority over all of Libya any time soon, so his continued campaign may only prolong the country’s instability.