original: He was holidaying in Germany the following summer when World War I broke out.
simplifications: He was on vacation in Germany the following summer when World War I broke out. He was on vacation in Germany when World War I began. The next summer, he was holidaying in Germany when World War I started. He was on vacation in Germany the following summer. World War I broke out. He was on his vacation in Germany the following summer when World War I occurred. He was on vacation in Germany when World War I started. He was vacationing in Germany the next summer when World War I started. The next summer he was on holiday in Germany when World War I broke out. He was having a summer vacation in Germany when the World War I broke out. World War I started when he was on Holiday in Germany the following summer.