After leaving Winchester College in 1942, Trevelyan served in the Second World War in the Rifle Brigade and was sent first to Algiers and then to Italy. On 23 May 1944, during the breakout from Anzio, the Green Howards battalion to which he was attached lost 230 men, some of whose deaths he blamed on himself. Towards the end of 1944, back with the Rifle Brigade, Trevelyan got a job in the British military mission in Rome, where he remained for two years, falling in love with central Italy. His participation in the bloody Battle of Anzio, in which he was wounded twice, was the subject of two memoirs. "The Fortress: A Diary of Anzio and After" (1957), was recognized for his painfully honest account of how a soldier responds to the terror of being under fire. Later, with the help of German and Italian friends, Trevelyan wrote "Rome ’44: The Battle for the Eternal City" (1981), which provided a vivid account of what it had been like for soldiers and civilians on the other side of the conflict.