Complex sentence: The Region is mainly responsible for defending the People's Republic of China from Mongolia and Russia, and also protects the capital of Beijing, with the largest number of military personnel of any of the seven regions.
Split into simpler sentences: The Region is mainly responsible for defending the People's Republic of China from Mongolia and Russia, and also protects the capital of Beijing. Regardless, the Beijing Military Region maintains the largest number of military personnel of any of the seven regions.