Complex sentence: The pre-war British Army was trained and equipped to be a small mechanized professional army however its main function was to garrison the British Empire and as became evident during the war was woefully unprepared and ill-equipped for a war with multiple enemies on multiple fronts.
Split into simpler sentences: The pre-war British Army was trained and equipped to be a small mechanized professional army. However, its main function was to garrison the British Empire and as became evident during the war, was woefully unprepared and ill-equipped for a war with multiple enemies on multiple fronts.