Complex sentence: After two years of training, World War II broke out with the Invasion of Poland, and he served on the minesweeper O.R.P. "Rybitwa" defending the port of Gydnia and the Peninsula of Hel.
Split into simpler sentences: After two years of training, World War II broke out with the Invasion of Poland. He served defending the port of Gydnia and the Peninsula of Hel.