In May 1915, the Germans sank the ‘Lusitania’, a British ship. Of the 1198 passengers who died, 128 of these were American. This aroused great anger in Americans, and further attacks followed as in June the German government announced passenger ships would be sunk without warning. The sinking of the Lusitania was used in British propaganda to remind the US citizens of the lives that had been lost due to Germany, hence they should join the war to defeat them.
The Germans decided to return to the policy of unrestricted submarine attacks on shipping in January 1917. The Submarines were not able to sink well-organised military ships and this is probably why passenger ships were attacked. Diplomatic relations between Germany and America were broken off by Wilson when and American liner was sunk on 3 February. This act taken by Germany was a big gamble to try to relieve the stalemate situation on the Western front, but this was an important reason towards America becoming involved in the way.
That same month, Alfred Zimmerman sent orders to the German Ambassador in Mexico to try to enlist support against the US. Mexico would be rewarded with the opportunity to regain territory, supposedly theirs, which was now part of the US. The note was intercepted by the British secret service, and when it published in March, Americans were outraged and infuriated. This was another gamble, but far more reckless. The stalemate had left their army crippled so that they could not let the war simply go on and on, so this gamble was taken.
It is important to realise that Wilson’s personal sympathies were undoubtedly with Britain, and for this reason he would have secretly been ‘routing for the British, and looking for a chance to join the war on behalf of them.
The economic links between the USA and Britain were also of considerable importance, and a German victory would have been financially damaging for them. America’s involvement on behalf of Britain was clear when The US loaned two billion dollars to the Allies, and also many factories in the US had become dependent on the allied purchases of war equipment.
When Wilson spoke to congress explaining to the people of America why they had to go to war, he included the sinking of American and ‘friendly neutral’s (the British)’ vessels, ‘without warning and without thought,’ and asked how could they remain neutral when the peace of the world was at stake.
In summary, the main reasons for America’s involvement were the reintroduction of the sinking of ships without warning, and the foolish Zimmerman telegraph. The sinking of the Lusitania, the economic situation, and Wilson’s own views resulted in America entering the war.