He lectured for a year at the University of Glasgow, then returned to London to work as a freelance writer and historian.
At the same time, he published three books in four years: 1939, The Making of the Second World War, Anthony Eden: A Biography, and The "X" Documents, The Secret History of Foreign Office Contacts with the German Resistance 1937-1939.
After a stay of 12 years in London, Aster returned to Canada in 1976 to take up a position with the Department of History, University of Toronto.
[citation needed] During this period, Aster continued to publish extensively on appeasement and revisionism, Neville Chamberlain, and the origins of the Second World War.
His latest publications include Appeasement and All Souls: A Portrait with Documents, 1937-1939, Dealing with Josef Stalin: The Moscow Blue Book, 1939, and a biography titled Power, Policy and Personality: The Life and Times of Lord Salter, 1881–1975.