On this day in history, the United States officially declared war on Austria-Hungary by a joint resolution of Congress (Sess. 2, ch. 1, 40 Stat. 429). The U.S. had already declared war on Germany in April, thus officially entering World War I. It never declared war on the other two Central Powers, the Ottoman Empire and Bulgaria.
Wilson justified his request for a declaration of war against Austria-Hungary, and his omission of a similar request regarding Turkey and Bulgaria, in a statement to Congress on December 4, saying:
Austria-Hungary is for the time being not her own mistress but simply the vassal of the German Government. We must face the facts as they are and act upon them without sentiment in this stern business. The Government of Austria-Hungary is not acting upon its own initiative or in response to the wishes and feelings of its own peoples, but as the instrument of another nation. We must meet its force with our own and regard the Central Powers as but one. The war can be successfully conducted in no other way. The same logic would lead also to a declaration of war against Turkey and Bulgaria. They also are the tools of Germany. But they are mere tools and do not yet stand in the direct path of our necessary action. (Congressional Record, December 4, 1917)
You can read his entire speech of December 4 here.