Complex sentence: The French planes, however, were not up to the enemy aircraft, the slow Potez 540 rarely survived three months of air missions, moving some 80 knots against enemy fighters flying at more than 250 knots.
Split into simpler sentences: The French planes, however, were not up to the enemy aircraft. The slow Potez 540 rarely survived three months of air missions, moving some 80 knots against enemy fighters flying at more than 250 knots.