How did Wilson justify going to war?

Answer 1

President Wilson pointed to Germany's actions in early 1917 that had angered the United States- in particular the Zimmerman Telegram, and Germany's use of unrestricted submarine warfare.

• The Zimmerman Telegram was secretly sent by Germany to the Mexican government. Long story short, Germany said that they expected to be at war with the US soon, and that if Mexico would join Germany's side and attack the US, Germany would send them help, and would make sure Mexico got back land taken by the US in the mid-1800s (like Texas, New Mexico and Arizona). British spies intercepted the telegram and gave it to the US.
• Early in the war, the mighty British Navy blockaded Germany, so Germany used submarines to attack ships in the Atlantic Ocean, and sometimes sank ships belonging to countries that weren't in the war, including the US. When these countries complained, Germany stopped doing so. But the British blockade remained and was hurting Germany. So Germany started attacking ships again.

The submarine stuff was the straw that broke the camel's back, so to speak. Wilson went before Congress and requested a declaration of war. He claimed that Germany was trying to conquer Europe, and that the US had a moral obligation to join the Allies to stop this. He went on to claim that "the world must be made safe for democracy", and mentioned the new republic that had formed in Russia earlier in 1917, as an example of the positive spread of democracy in the world.

(of course, a few months later the Bolsheviks took control of Russia, sparking a horrible civil war and eventually a ruthless Dictatorship)

Answer 2

Un-restricted submarine warfare (sinking anything in sight without warning).