 With the start of World War I in 1914, Buchanan was joined up with the Grenadier Guards as a second lieutenant in August of that year.In September 1917 he was still serving in the Grenadier Guards, by this time he held the full rank of lieutenant and the temporary rank of acting captain, it was in this month that he was mentioned in dispatches in the "London Gazette" as having been awarded the Military Cross.In June 1919, Buchanan was awarded the Distinguished Service Order.