On the afternoon of May 1, President Trump’s son-in-law, Jared Kushner, welcomed a high-level delegation of Saudis to a gilded reception room next door to the White House and delivered a brisk pep talk: “Let’s get this done today.”

Mr. Kushner was referring to a $100 billion-plus arms deal that the administration hoped to seal with Saudi Arabia in time to announce it during Mr. Trump’s visit to the kingdom this weekend.