“Casus belli” is a Latin expression meaning the justification for acts of war.
This message was declared by President Woodrow Wilson to the people of the United States on August 4th 1914. Even though, Bulgaria and Turkey joined Germany and Austria-Hungary in the Central Powers and France, England, Russia joined forces to form the Allied Powers earlier in the month- there was no, “Casus belli” and the United States would remain neutral.
A short time after the “deceleration of neutrality” on May 7th 1915, the British ocean liner Lusitania was Torpedoed by Germans without warning and sunk off the coast of Ireland killing 1,198 passengers of those, 128 were Americans. Wilson protested this attack by threatening to sever diplomatic relation with Germany. Many nations across the world believed this would thrust the United States into a declaration of war- they remained neutral.
It was in the interests of the British to keep US passions inflamed, and a fabricated story was circulated that in some regions of Germany, schoolchildren were given a holiday to celebrate the sinking of the Lusitania. This story was so effective that James W. Gerard, the US ambassador to Germany, recounted it in his memoir of his time in Germany, Face to Face with Kaiserism (1918), though without substantiating its validity. 
January 22nd,1917, President Woodrow Wilson advocated "Peace Without Victory" in an attempt to cease fire from both sides. Wilson knew that fighting would never stop- regardless if Allies or the Central Powers were victorious. Wilson plead for all of the nations to stop fighting with no one side being the victor in the war, or "Peace without Victory".
Soon after on March 1st the “tides turned” when a telegram, sent on January 16th, intercepted by British intelligence from Germany's Secretary of State, Arthur Zimmermann, sent to German ambassador in Washington, Johann von Bernstorff, was made public. The telegram advised, “We [Germany] intend to begin on the first of February unrestricted submarine. We shall endeavor in spite of this to keep the United States of America neutral. In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona . The citizens of the United States were outraged and Wilson was now forced to change his previous stance on the War.
On April 6th, 1917, with the words of Woodrow Wilson echoing throughout the hall from days earlier, “There are, it may be, many months of fiery trial and sacrifice ahead of us. It is a fearful thing to lead this great peaceful people into war, into the most terrible and disastrous of all wars, civilization itself seeming to be in the balance. But the right is more precious than peace, and we shall fight for the things which we have always carried nearest our hearts -- for democracy, […] To such a task we can dedicate our lives and our fortunes, everything that we are and everything that we have, with the pride of those who know that the day has come when America is privileged to spend her blood and her might for the principles that gave her birth and happiness and the peace which she has treasured. God helping her, she can do no other. “
The House of Representatives voted 373 to 50, and the Senate voted in favor by 82 to 6 in favor of declaring war
And so, “Casus belli” prevailed after all..
 Quinn, Patrick J. (2001). The Conning of America. Rodopi. pp. 54–55
 The Zimmermann Telegram
 Woodrow Wilson, War Messages, 65th Cong., 1st Sess. Senate Doc. No. 5, Serial No. 7264, Washington, D.C., 1917; pp. 3-8, passim.