Complex sentence: The 5th Canadian Infantry Brigade was mobilized on 1 September 1939 part of 2nd Canadian Infantry Division, even before the declaration of war, and the battalions were promptly fleshed out by volunteers.
Split into simpler sentences: The 5th Canadian Infantry Brigade was mobilized on September 1,1939 as part of the 2nd Canadian Infantry Division. The brigade was formed before the declaration of World War II, and the battalions were promptly fleshed out by volunteers.