Complex sentence: The forces that had escaped to the United Kingdom were slowly built up over the next few years and on D-Day June 6th, 1944 the Royal Norwegian Navy attached to the invasion of Normandy numbered ten ships and 1,000 sailors.
Split into simpler sentences: The forces that had escaped to the United Kingdom were slowly built up over the next few years. On D-Day (June 6,1944), the Royal Norwegian Navy attached to the invasion of Normandy numbered ten ships and 1,000 sailors.