Complex sentence: In 1917, when the US entered World War I, he enlisted in the Canadian Expeditionary Force, saw combat in the trenches in France with the Gordon Highlanders, and was undergoing flight training in the fledgling Royal Air Force (RAF) when the war ended.
Split into simpler sentences: In 1917, when the United States entered World War I, he enlisted in the Canadian Expeditionary Force. He saw combat in the trenches in France with the Gordon Highlanders and was undergoing flight training in the fledgling Royal Air Force (RAF) when the war ended.