Complex sentence: Rebel pilots often mistakened it for a miniature Star Destroyer when it was accompanied by Star Destroyers; it was only when they tried to escape into hyperspace that they realized it was an Interdictor.
Split into simpler sentences: Rebel pilots often mistook it for a miniature Star Destroyer when it was accompanied by Star Destroyers. It was only when they tried to escape into hyperspace that they realized it was an Interdictor.