Complex sentence: Cowie convinced the Sultan that to achieve a lasting success in overcoming the Spanish blockade he would need access to a safe harbour; from there he could wait until the shipping route to Sulu was free from Spanish warships.
Split into simpler sentences: Cowie convinced the Sultan that to achieve a lasting success in overcoming the Spanish blockade would only be guaranteed if he could get access to a safe harbour. From there he could wait until the shipping route to Sulu was free from Spanish warships.