Complex sentence: The Roman Republic had never been much of a seafaring nation, but it had to learn, and learn fast, in the Punic Wars with Carthage, and developed the technique of grappling and boarding enemy ships with soldiers.
Split into simpler sentences: The Roman Republic had never been much of a seafaring nation, but it had to learn, and learn fast. In the Punic Wars with Carthage, Romans developed the technique of grappling and boarding enemy ships with soldiers.