Complex sentence: Meanwhile in the English civil war, Oliver Cromwell united his country into the Commonwealth of England and in a few years created a powerful navy, expanding the number of ships and greatly improving organisation and discipline positioning England to mount a global challenge to Dutch trade dominance.
Split into simpler sentences: Meanwhile in the English civil war, Oliver Cromwell united his country into the Commonwealth of England. In a few years he created a powerful navy by expanding the number of ships and greatly improving its organisation and discipline, thereby positioning England to mount a global challenge to Dutch trade dominance.