Capacity duel: Southeast Asia air cargo competes with China freight for space on trans-Pacific flights.(Special report: Southeast Asia)

On Dec. 10, United Airlines expanded its service to include a Southeast Asian nation where regular flights by a U.S. airline once would have been inconceivable. United's new service to Ho Chi Minh City, Vietnam, reflects changed political attitudes and growing prospects for trade.

Technically, the new flight reaches the U.S. shortly after departure from Vietnam. It takes off at 6:20 a.m. Vietnam time and lands in San Francisco at 8:30 a.m. the same day, but that's because it crosses the international dateline in an eastbound direction. Actual transit time is just over 18 hours, including a stopover in Hong Kong.

Because of the long distance from Southeast Asia to North America, stops at gateways such as Hong Kong, Taipei, Seoul or Tokyo are the norm. "There are no direct routes. You always go over one hub or another," said Chris Coppersmith, president and chief executive of Target Logistics Services in Compton, Calif.

Many airlines maximize revenue by filling the intra-Asian segment of their routes with cargo moving within the region, and using the trans-Pacific segment for cargo originating near …