a prelude to (something)
An introductory event or action. The economic troubles of these companies were just a prelude to the global financial meltdown that would the following year. We regard these acts of aggression as a prelude to war.
prelude to something
an act or event that comes before and signals another act or event. Her rudeness to her boss was a prelude to her resignation. The Munich Pact was a prelude to World War II.