Complex sentence: Warmwell had been allocated for use by American fighter units in August 1942 but was not taken up at that time and RAF units continued in residence, albeit that American Spitfire and Thunderbolt squadrons occasionally made use of the airfield as a forward or transit base.
Split into simpler sentences: Warmwell had been allocated for use by American fighter units in August 1942 but was not taken up at that time and RAF units continued in residence. USAAF Spitfire and Republic P-47 Thunderbolt squadrons occasionally made use of the airfield as a forward or transit base.