Despite the shocking news of Zimmerman`s telegram, Wilson was still reluctant to call for a declaration of war. He waited until March 20 before calling a cabinet meeting to raise the issue — nearly a month after seeing the telegram for the first time. The exact reasons for Wilson`s decision to choose war in 1917 remain debated among historians, especially given his efforts to avoid war in 1915 following the sinking of the British passenger ships Lusitania and Arabic, which resulted in the deaths of 131 Americans. Chancellor Theobald von Bethmann Hollweg (1856-1921) issued the Sussex Pledge on 4 May 1916. Germany has promised to stop attacking passenger ships, extending the promise made in the Arab promise. Merchant ships would only be sunk if war material was on board, but only after all passengers, including the crew, had left the ship. This policy of appeasement of the United States supported the German war effort. German submarines effectively sank large quantities of purely military ships over the next six months, successfully avoiding any confrontation with the United States until they resumed unconditional submarine warfare in January 1917. Stunned by the news, President Wilson appeared before Congress on February 3 to announce that he had severed diplomatic relations with Germany. However, he refrained from calling for a declaration of war because he doubted that the American public would support him unless there was sufficient evidence that Germany intended to attack American ships without warning. Wilson left open the possibility of negotiating with Germany if its submarines refrained from attacking American ships. Nevertheless, in February and March 1917, German submarines targeted and sank several American ships, and many American passengers and sailors died.
After the resumption of unrestricted submarine warfare in Germany on February 1, 1917, countries attempted to limit or even abolish submarines. Instead, the London Declaration required submarines to abide by pricing rules. These regulations did not prohibit the arming of merchants, but when they reported contact with submarines (or thieves), they became de facto naval auxiliaries and the protection of price rules was lifted.  This has made restrictions on submarines virtually unnecessary.  Although such tactics increase the submarine`s combat power and improve its chances of survival, some consider them a violation of the rules of war, especially when used against neutral ships in a war zone. On April 2, 1917, President Woodrow Wilson appeared before a joint session of Congress to demand a declaration of war against Germany. Wilson cited Germany`s violation of its promise to suspend unfettered submarine warfare in the North Atlantic and Mediterranean, and its attempts to lure Mexico into an alliance against the United States, as its reasons for declaring war. On April 4, 1917, the U.S. Senate voted in favor of the measure to declare war on Germany.
The House agreed two days later. The United States declared war on Austria-Hungary on December 7, 1917. Germany`s resumption of submarine attacks on passenger and merchant ships in 1917 was the main motivation for Wilson`s decision to lead the United States into World War I. After the sinking of an unarmed French ship, the Sussex, in the English Channel in March 1916, Wilson had threatened to sever diplomatic relations with Germany if the German government did not attack all passenger ships and allow the crews of enemy merchant ships to escape their ships from any attack. On May 4, 1916, the German government had agreed to these terms and conditions in what became known as the “Sussex Promise.” All four cases were attempts to impose a naval blockade on countries, especially those that rely heavily on the merchant navy to supply their war industry and feed their populations (such as Britain and Japan), when their enemies were unable to establish a conventional naval blockade. On January 31, 1917, Bethmann Hollweg appeared before the German Reichstag government and announced that unrestricted submarine warfare would resume the next day, February 1: “The destructive plans of our adversaries cannot be expressed more strongly. We were challenged to fight to the end. We accept the challenge. We put everything in place and we will be victorious. German Chancellor Theobald von Bethmann-Hollweg protested the move, arguing that the resumption of submarine warfare would drag the United States into the war on behalf of the Allies. This, he argued, would lead to Germany`s defeat. Despite these warnings, the German government decided to resume unrestricted submarine attacks against all Allied and neutral ships in the prescribed war zones, as it assumed that German submarines would end the war long before the arrival of the first American troop carrier in Europe.
As a result, the German ambassador to the United States, Count Johann von Bernstorff, made a presentation on the 31st. In January 1917, he issued a note to U.S. Secretary of State Robert Lansing declaring Germany`s intention to resume submarine warfare without restriction the next day. Since the beginning of the First World War in 1914, Germany has conducted a highly effective submarine campaign against the merchant navy. This campaign intensified during the war and almost succeeded in bringing Britain to its knees in 1917. .