Complex sentence: He became involved in a project to control the Strait of Magellan by building forts at its mouth, but the project was abandoned when it was realized that many ships would simply bypass the Strait by sailing round Cape Horn.
Split into simpler sentences: He became involved in a project to control the Strait of Magellan by building forts at its mouth. However, the project was abandoned when it was realized that many ships would simply bypass the Strait by sailing round Cape Horn.