Vietnam's warming relations with the U.S. appear set to hit a frosty patch after a Hanoi court Wednesday sentenced a prominent U.S.-trained human rights lawyer, one of the country's best-known bloggers, to 30 months in prison in a tax-evasion case widely viewed to be politically motivated.

The former wartime foes have developed a growing commercial and military relationship in recent years, culminating in Vietnamese President Truong Tan Sang's visit to the White House to meet President Barack Obama in July. But human rights...