Title: Allies of World War I
Content:  The United States declared war on Germany in April 1917 on the grounds that Germany violated US neutrality by attacking international shipping with its unrestricted submarine warfare campaign. The remotely connected Zimmermann Telegram of the same period, within which the Germans promised to help Mexico regain some of its territory lost to the U.S nearly seven decades before in the event of the United States entering the war, was also a contributing factor. The US entered the war as an "associated power", rather than a formal ally of France and the United Kingdom, in order to avoid "foreign entanglements". Although the Ottoman Empire and Bulgaria severed relations with the United States, neither declared war, nor did Austria-Hungary. Eventually, however, the United States also declared war on Austria-Hungary in December 1917, predominantly to help hard-pressed Italy.
Title: United States declaration of war on Austria-Hungary
Content:  On April 6, 1917 the United States declared war against Germany. The declaration of war was enacted at the request of US President Woodrow Wilson by a vote of both chambers of the US Congress, with the US House of Representatives voting 373 to 50 (nine not voting) in favor of war and the US Senate voting 82 to six. In his speech to Congress requesting for war to be declared against Germany, Wilson addressed the question of Austria-Hungary, an ally of Germany: "that government [Austria-Hungary] has not actually engaged in warfare against citizens of the United States on the seas, and I take the liberty, for the present at least, of postponing a discussion of our relations with the government in Vienna." Two days later, Austria-Hungary terminated diplomatic relations with the United States and requested, in a diplomatic note that was delivered to the chargé d'affaires of the US embassy in Vienna, for American diplomats to depart the country.
Title: American entry into World War I
Content:  The United States Senate, in a 74 to 0 vote, declared war on Austria-Hungary on December 7, 1917, citing Austria-Hungary's severing of diplomatic relations with the United States, its use of unrestricted submarine warfare and its alliance with Germany. The declaration passed in the United States House of Representatives by a vote of 365 to 1. President Wilson also came under pressure from Senator Henry Cabot Lodge, and from former President Theodore Roosevelt, who demanded a declaration of war on the Ottoman Empire and Bulgaria, as Germany's allies. President Wilson drafted a statement to Congress in December 1917 which said "I... recommend that Congress immediately declare the United States in a state of war with Austria-Hungary, with Turkey and with Bulgaria". However, after further consultations, the decision to go to war against Germany's other allies was postponed.
Title: History of U.S. foreign policy, 1913–1933
Content:  to pay for military expenses, loans to Allied governments, and increased industrial and agricultural production. The declaration of war by the United States against Germany passed Congress by strong bipartisan majorities on April 6, 1917, with opposition from ethnic German strongholds and remote rural areas in the South. The United States would also later declare war against Austria-Hungary in December 1917. The U.S. did not sign a formal alliance with Britain or France but operated as an "associated" power—an informal ally with military cooperation through the Supreme War Council in London. Generals Frederick Funston and Leonard Wood had been contenders for the command of American army forces in Europe, but Funston 
Title: United States in World War I
Content:   The United States declared war on Germany on April 6, 1917, nearly three years after World War I started. A ceasefire and Armistice was declared on November 11, 1918. Before entering the war, the U.S. had remained neutral, though it had been an important supplier to the United Kingdom, France, and the other powers of the Allies of World War I. The U.S. made its major contributions in terms of supplies, raw material, and money, starting in 1917. American soldiers under General of the Armies John Pershing, Commander-in-Chief of the American Expeditionary Force (AEF), arrived at the rate of 10,000 men a day on the Western Front in the summer of 1918. 
Title: United States declaration of war on Germany (1917)
Content:  "WHEREAS, The Imperial German Government has committed repeated acts of war against the people of the United States of America; therefore, be it resolved by the Senate and House of Representatives of the United States of America in Congress assembled, That the state of war between the United States and the Imperial German Government, which has thus been thrust upon the United States, is hereby formally declared; and that the President be, and he is hereby, authorized and directed to employ the entire naval and military forces of the United States and the resources of the Government to carry on war against the Imperial German Government; and to bring the conflict to a successful termination all the resources of the country are hereby pledged by the Congress of the United States."
Title: Presidency of Woodrow Wilson
Content:  draft to raise the army, increased taxes to pay for military expenses, loans to Allied governments, and increased industrial and agricultural production. The declaration of war by the United States against Germany passed Congress by strong bipartisan majorities on April 6, 1917, with opposition from ethnic German strongholds and remote rural areas in the South. The United States also later declared war against Austria-Hungary in December 1917. The U.S. did not sign a formal alliance with Britain or France but operated as an "associated" power—an informal ally with military cooperation through the Supreme War Council in London. Generals Frederick Funston and Leonard Wood had been contenders for the command of American army forces in 
Title: United States declaration of war on Germany (1917)
Content:   On April 2, 1917, President Woodrow Wilson asked a special joint session of the United States Congress for a declaration of war against the German Empire. Congress responded with the declaration on April 6.
Title: The Great War: American Front
Content:  the incident drawing the European powers into a whirlwind of war. At the eruption of an alternate World War I known as the Great War, the USA and CSA find themselves on opposite sides of the divide between the Central Powers and the Entente Powers, respectively. The fighting in Europe quickly spreads to North America, where the pro-German United States under President Theodore Roosevelt and 75-year-old General George Armstrong Custer declare war on President Woodrow Wilson's CSA, which are allied with the United Kingdom, France, and Russia. After the Confederate seizure of Washington D.C. and invasion into Pennsylvania, and initial US invasions of Kentucky, Canada, and western Virginia, the conflict bogs down into 
Title: Italian declaration of war on the United States
Content:  On December 11, 1941, Italy declared war on the United States in response to the latter's declaration of war upon the Empire of Japan following the attack on Pearl Harbor four days earlier. Germany also declared war on the U.S. the same day. The US immediately responded by declaring war on Germany and Italy, thus thrusting the United States in fighting two major fronts across the Pacific and Atlantic Oceans in World War II.

Question: who did america declare war on in ww1?
Answer: Austria-Hungary.