Complex sentence: However, these hopes were quickly dashed with the emergence of the Cold War as a result of underlying ideological differences and mutual distrust between the Soviet Union and the United States of America leading to an arms race which ultimately became a nuclear arms race.
Split into simpler sentences: However, these hopes were quickly dashed with the emergence of what became known as the Cold War as a result of underlying ideological tensions largely between the Soviet Union and the United States of America. Mutual distrust between the West and the new Eastern Bloc resulted in an arms race which ultimately became a nuclear arms race.