The 1918–1920 influenza pandemic remains the deadliest influenza pandemic in recorded history. It started in the midst of World War I and killed an estimated 50–100 million people worldwide, many from complications of pneumonia. Approximately 500 million, or one-third of the world's population, became infected. In the United States, an estimated 850,000 died. The exceptionally contagious, unknown strain of influenza virus spread rapidly and attacked all ages, but it especially targeted young adults (ages twenty to forty-four). This essay examines the evolution of four waves of the 1918–1920 influenza pandemic, emphasizes the role of the U.S. Navy and sea travel as the initial transmitters of the virus in the United States, and focuses on California communities and military installations as a case study in the response to the crisis. Although the world war, limited medical science, and the unknown nature of the virus made it extremely difficult to fight the disease, the responses of national, state, and community leaders to the 1918–1920 influenza pandemic can provide useful lessons in 2020, as the onslaught of COVID-19 forces people worldwide to confront a terrible illness and death.