Complex sentence: Assured by a travel agency that there would be no war he visited North America with his mother in 1939 to visit the World's Fair in New York City and found himself visiting Niagara Falls, Ontario when World War II broke out.
Split into simpler sentences: Assured by a travel agency that there would be no war, he visited North America with his mother in 1939 to attend the World's Fair in New York City. He was visiting Niagara Falls, Ontario when World War II broke out.