What was one of the causes for the U.S. entry into World War I?

Unrestricted submarine warfare was a significant factor that led to the United States' entry into World War I. This warfare policy, primarily implemented by Germany, involved attacking merchant ships without warning, which threatened American lives and commerce. The most notable incident was the sinking of the RMS Lusitania in 1915, which resulted in the deaths of 128 Americans. This aggressive tactic angered the American public and government, leading to heightened tensions and ultimately contributing to the decision to join the conflict in 1917.

While imperialism and economic sanctions played their roles in the broader context of global tensions, they were not direct causes of U.S. involvement in World War I. Additionally, attacks on Pearl Harbor occurred much later, in World War II, and are unrelated to this question.