When Thomas Wong set foot in the United States Embassy in Beijing this summer for a new diplomatic posting, it was vindication after years of battling the State Department over a perceived intelligence threat — himself. Diplomatic Security officers had informed him when he joined the foreign service more than a decade ago that they were banning him from working in China. In a letter, he said, they wrongly cited the vague potential for undue “foreign preference” and suggested he could be vulnerable to “foreign influence.” Mr. Wong had become…