President Donald Trump has found a new purpose for the massive triumphal arch he wants to build near Arlington National Cemetery: national security. “At the strong request of the United States Military, and for National Security purposes, I have agreed to convert the magnificent Triumphal Arch, planned since the Civil War Era many years ago, at the Receptive Circle adjoining the Arlington Memorial Bridge, into a top grade Military Complex/Triumphal Arch,” Trump wrote Sunday on Truth Social.  Trump said the proposed 250-foot structure would house and rapidly deploy “large numbers of drones,” position snipers on its roof and plaza and store “large quantities of sniper ammunition.” He also claimed Washington is the only one of the world’s “59 top cities and Capitals” without a triumphal arch. But the post raises questions about both the history of the project and its newly announced military purpose. The Pentagon has not publicly identified which part of the military requested the conversion or what security threat the facility would address. A military official said the Defense Department had …