American industry prospered off trade with the Allies. Germany and the Central Powers protested American trading with the Allies, although America wasn't breaking the international neutrality laws -- Germany was free to trade with the U.S., but Britain prevented this trade by controlling the Atlantic Ocean by which Germany had to cross in order to trade with the U.S.
In 1915, several months after Germany started to use submarines in the war, one of Germany's submarines sunk the British liner Lusitania, killing 128 Americans.
Americans demanded war but President Wilson stood strong on his stance against war. When Germany sunk another British liner, the Arabic, in 1915, Berlin agreed to not sink unarmed passenger ships without warning. Germany continued to sink innocent ships as apparent when one of its submarines sank a French passenger steamer, the Sussex. President Wilson informed the Germans that unless they renounced the inhuman practice of sinking merchant ships without warning, he would break diplomatic relations, leading to war. Germany agreed to Wilson's ultimatum, but attached additions to their Sussex pledge: the United States would have to persuade the Allies to modify what Berlin regarded as their illegal blockade. Wilson accepted the Germany pledge, without accepting the "string" of additions.
Wilson Wins the Reelection in 1916
The Progressive Party and the Republican Party met in 1916 to choose their presidential candidate. Although nominated by the Progressives, Theodore Roosevelt refused to run for president. The Republicans chose Supreme Court justice Charles Evans Hughes. The Republican platform condemned the Democratic tariff, assaults on the trusts, and Wilson's dealings with Mexico and Germany.
The Democrats chose Wilson and ran an anti-war campaign. Woodrow Wilsonwon the election of 1916 and was reelected to the presidency.