LONDON—Two people smugglers were found guilty on Monday of the manslaughter of 39 Vietnamese men, women, and children who suffocated to death in the back of a refrigerated truck as they tried to make their way to Britain. The discovery of so many dead people—some as young as 15—shocked Britain and Vietnam, and shone a spotlight on the illicit global trade that sends the poor of Asia, Africa, and the Middle East on perilous journeys to the West. As oxygen levels fell in the back of the truck, some tried desperately to escape, but in vain. Others used mobile phones to say their last farewells to devastated relatives on the other side of the world. “This is an unimaginably tragic case: 39 vulnerable people desperate for a new life were driven to put their trust in a network of unscrupulous people smugglers,” said Russell Tyner, a prosecutor in the Organised …