(A) The Zimmerman Telegram
(B) Unrestricted Submarine Warfare
(C) The Russian Revolution
(D) ** Assassination of Archduke Franz Ferdinand
EXPLANATIONS BELOW
Concept note-1: -The assassination set off a rapid chain of events, as Austria-Hungary immediately blamed the Serbian government for the attack. As large and powerful Russia supported Serbia, Austria asked for assurances that Germany would step in on its side against Russia and its allies, including France and possibly Great Britain.
Concept note-2: -What were the main reasons for U. S. involvement in the war? The main reasons the US got involved in the war was because of nationalism, imperialism, militarism, and forming allies.
Concept note-3: -The US entered World War I because Germany embarked on a deadly gamble. Germany sank many American merchant ships around the British Isles which prompted the American entry into the war.