Update: The United States and South Korea formally announced the trade agreement in a joint statement on Wednesday, and said that it “represents important progress in improving U.S.-Korea trade and economic relations.”

WASHINGTON — President Trump scored his first significant trade deal this week, securing a pact with South Korea that represents the type of one-on-one agreement that Mr. Trump says makes the best sense for American companies and workers.

The deal, which is expected to be formally announced on Wednesday, opens the South’s market to American autos by lifting existing limits on manufacturers like Ford Motor and General Motors, extends tariffs for South Korean truck exports and restricts, by nearly a third, the amount of steel that the South can export to the United States. Mr. Trump used his threat of stiff steel and aluminum tariffs as a cudgel to extract the concessions he wanted, helping produce an agreement that had stalled amid disagreements this year.

But winning the deal may have had more to do with the geopolitical realities confronting the United States and South Korea as America embarks on tricky nuclear discussions with North Korea. The United States cannot afford a protracted trade standoff at a moment when it needs the South as an ally.