After more than a year of debate about whether a Chinese company’s plan to build a corn mill in North Dakota was an economic boon or a geopolitical risk, an assistant secretary of the Air Force has weighed in with a warning that the “project presents a significant threat to national security.” The letter from Assistant Secretary Andrew P. Hunter, released publicly on Tuesday by North Dakota’s senators, noted the proximity of Grand Forks Air Force Base to the proposed mill and said the project raised “near- and long-term risks…