BILLINGS, Mont. (AP) — A brewing trade war between China and the U.S. comes at an inopportune time for Montana ranchers, who are seeking to close a multi-year deal to export up to $200 million of beef through China's largest online retailer, industry representatives said Thursday.

The export agreement between the Montana Stockgrowers Association and Chinese e-commerce giant JD.com was touted as a landmark agreement when it was announced in November, soon after the lifting of longstanding restrictions on Chinese imports of U.S. beef.

But suddenly looming over the export partnership is the fast-...