Complex sentence: The War Office had originally selected the Tetrarch light tank for use by the airborne forces, but it had not been designed with that exact purpose in mind; so the War Office believed that a purpose-built tank would be required to replace the Tetrarch.
Split into simpler sentences: The War Office had originally selected the Tetrarch light tank as the tank to be utilized by the airborne forces, but it was not designed with this exact purpose in mind. However, the War Office believed that a purpose-built airborne light tank was required to eventually replace the Tetrarch.