Complex sentence: Born in Amsterdam, he was 16 when he applied for the Marines and was accepted to the bootcamp in Doorn in July 1949, in the summer of 1950, the Korean War started, and he applied as a volunteer in the van Heutz regiment.
Split into simpler sentences: Born in Amsterdam, he was 16 when he applied for the Marines and was accepted to the bootcamp in Doorn in July 1949. Just one year later, in the summer of 1950, the Korean War started, and he applied as a volunteer in the van Heutz regiment.