Complex sentence: Britain at the time was heavily engaged in the Boer War, consequently a large part of the British army was tied down in South Africa and deploying large numbers of troops from its garrisons in India would take too much time and weaken internal security there.
Split into simpler sentences: Britain at the time was heavily engaged in the Boer War, and, consequently, a large part of the British army was tied down in South Africa. Deploying large numbers of troops from British garrisons in India would take too much time and weaken internal security there.