Abstract Since the 1880s, with the improvement of the maritime power of the US, its diplomatic strategy towards Chile has accordingly changed. Due to the contest of commercial interests between Chile and the US, the US mediation of the South American Pacific War was obviously biased towards Peru and Bolivia. Furthermore, it indirectly participated in the Chilean civil war, secretly provided assistance to the Balmaceda administration, and even sent the Navy to directly participate in the evacuation of political exiles. The essence of the opposition between the two sides lies in the following fact: the US is dissatisfied with Chile's exclusive possession of saltpeter, making full use of the instability of the saltpeter economy which has been a drag to the construction of Chilean navy, and hyping up the “Baltimore Incident” for political gains against the context of the collapse of international saltpeter market and the reversal of the two countries’ naval strengths. For the first time, the US Navy made a public attack on Chile, forcing the latter to compromise. After the incident, “Monroe doctrine” was no longer just a “rumor” of taking a free ride on the British navy's safe and secure train, which laid a solid foundation for the formation of the US maritime power in the Western Hemisphere. Therefore, it is particularly necessary to understand the precedent of the US Navy's intervention in the Chilean judicial trial in the Baltimore Incident 100 years ago, which provides clues to understand the US naval compulsory diplomacy in the process of the South China Sea arbitration.