What event was the catalyst for the United Stated to entered the war

answer:

Two events forced the U.S. to join the war. The first was Germany's unrestricted submarine warfare. On May 7, 1915, a German submarine sunk the British ocean liner RMS Lusitania, killing 1,198 passengers including 128 Americans. This was the catalyst that changed public opinion on America's neutrality.Jan 17, 2018

Explanation:

the berlin blockade was an international crisis that arose from an attempt by the soviet union, in 1948–49, to force the western allied powers (the united states, the united kingdom, and france) to abandon their post-world war ii jurisdictions in west berlin. in march 1948 the allied powers decided to unite their different occupation zones of germany into a single economic unit. in protest, the soviet representative withdrew from the allied control council. coincident with the introduction of a new deutsche mark in west berlin (as throughout west germany), which the soviets regarded as a violation of agreements with the allies, the soviet occupation forces in eastern germany began a blockade of all rail, road, and water communications between berlin and the west. on june 24 the soviets announced that the four-power administration of berlin had ceased and that the allies no longer had any rights there.