When war broke out in 1914, the United States of America was not directly affected by any of the issues that divided the European powers. President Woodrow Wilson was supported by most Americans when he declared the USA would be strictly neutral.
A large proportion of the population of the USA in 1914 were immigrants from Europe, or their children. They tended to support their various mother countries in the European war.
The USA quarrelled with Britain in the autumn of 1914 when the Royal Navy began stopping US ships from steaming to Germany. President Wilson moved quickly to avert an open dispute.
It was agreed that no goods likely to be useful to the war effort would be sent from America to Germany, but that peaceful trade could continue.
In February 1915 a more serious quarrel broke out with Germany. The German navy announced that from 4 February it would sink without notice any merchant ships heading to Britain.
On 10 February Wilson told Germany that he would declare war if any US ships were sunk. When the British passenger ship, Lusitania, was sunk and many Americans killed, Wilson began moves to break diplomatic relations. Germany called off the campaign.
Public opinion in America was outraged by the German execution of Nurse Edith Cavell. Stories of German behaviour in occupied areas also upset many Americans.
On 31 January 1917 the German ambassador told President Wilson that Germany was once again beginning unrestricted warfare on merchant ships, as of the next day. Wilson again said he would declare war if any US ships were sunk.
On 1 March, attempts by the German government to persuade Mexico to attack the USA were discovered. American public opinion turned firmly against Germany.
On 20 March a German U-boat sank an unarmed American merchant ship. On 2 April the USA declared war on Germany.
This is an extract from 1000 Facts on World War I by Rupert Matthews