With litigation threatening to derail President Trump’s plans for a 250-foot triumphal arch in Washington, two of his top aides on Wednesday submitted sworn statements to a federal court claiming that the arch was needed for national security purposes. Defense Secretary Pete Hegseth and Markwayne Mullin, the homeland security secretary, contended that even though national security was never invoked in the initial proposal for the arch, the project was now needed to keep the United States safe from drone attacks. “The arch is a generational national defense opportunity to mitigate these evolving threats,” Mr. Hegseth told the court. He added...