Complex sentence: The brigade was mobilised on 4 August 1914 at the outbreak of the First World War and concentrated in the Canterbury area of Kent in August 1914 where it remained (under Second Army of Central Force) until September 1915.
Split into simpler sentences: The brigade was mobilised on 4 August 1914 at the outbreak of the First World War and concentrated in the Canterbury area of Kent. The rest of the brigade remained in the Canterbury area (under Second Army of Central Force) until September 1915.