Complex sentence: The P-51 Mustang first entered squadron service in Europe with the British in early 1942, the Allison V-1710 engined P-51A (Mustang I) having much success with the RAF, although it found the aircraft's performance inadequate at higher altitudes.
Split into simpler sentences: The P-51 Mustang first entered squadron service in Europe with the British in early 1942. The Allison V-1710-engined P-51A (Mustang I) was successful in RAF service, although the British found the aircraft's performance inadequate at higher altitudes.