Born in St. George's, London and based in Plaistow, Essex, the Jewish Isaacs fought under the name Harry Reeve and made his professional debut in 1910.

He hit another winning streak in mid-1912, and despite a mixed record in 1913, got a shot at the vacant British middleweight title in February 1914 against Pat O'Keeffe, who won a points decision after 20 rounds.

[1][2][3] Reeve moved up to light-heavyweight and beat British heavyweight champion Joe Beckett in December 1914 in a non-title bout.

[citation needed] A run of eight straight wins in 1915 and 1916 led to a fight for Dick Smith's British light-heavyweight title on 30 October 1916 at the National Sporting Club at Covent Gardens.

[citation needed] Reeve served in the 7th Middlesex Regiment and Military Police during his service with the British Army in the First World War.