December 7 1917, Washington–The United States had been at war with Germany for eight months now, but as of yet had not declared war on her allies to the south. For the most part, they had not violated America’s rights on the high seas, and Wilson had held out hope for a separate peace with them. However, by late 1917 this prospect was appearing more distant, and the Italian collapse at Caporetto meant that American aid might soon be needed against the Austrians. Even if no American troops went to Italy, the United States was participating in the Supreme War Council whose remit included the Italian front. On December 4, Wilson addressed Congress for the first time since the declaration of war on Germany, asking for a declaration of war against Austria-Hungary.
The recent release of the Treaty of London by the Bolsheviks led to some concerns that the United States would be acting to dismember the Austro-Hungarian Empire, but Wilson successfully assuaged these, stating that he in no way wanted to “rearrange or impair” the empire, but that war against it was necessary while it served as a tool of the Germans. Others in Congress wanted to go further and declare war against Turkey and Bulgaria, but Wilson still held out hope of a negotiated peace with both countries, they had not acted against the United States in any way, and the United States would be highly unlikely to take offensive action against them even if they did declare war.
The war resolution cited the Austrians’ espionage and sabotage efforts in the United States in the first year of the war (which had led to the expulsion of their ambassador), as well as Austrian unrestricted submarine warfare the Mediterranean (even if most of their submarines had actually been German). On December 7, the House and Senate passed the declaration of war, and it was approved by Wilson that evening. There was only a single dissenting vote, from a Socialist congressman. Even Jeannette Rankin voted in favor, saying
“the vote we are now to cast is not a vote on a declaration of war. If it were, I should vote against it. This is a vote on a mere technicality in the prosecution of a war already declared." Notably absent during the vote was Robert LaFollette; whether he missed the vote on purpose or whether the vote was scheduled so that he would be absent is unclear.
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