It is the year 1914. World War 1 has begun after the Assasination of Archduke Franz Ferdinand. From 1914 - 1916 everything goes as normal as in our timeline. But when 1917 arrived, America started noticing that the British blockade was starving the German populace. Woodrow Wilson starts enforcing "Freedom of the Seas". This was ignored by the British.
Wilson authorizes USN ships to start escorting supply ships that has supplies for Germany. One of the US ships, USS Fanning (I think), is sunk while escorting ships. Anti British sentiments starts in America. With all this, and the memory of the Lusitania's meat shields, joins Germany in 1917.