England, sinking ships without warning.  This was in violation of international law.  It also angered the US because it led to the sinking of American ships.  The Germans pledged to stop doing this, but later rescinded the pledge.  This was the direct cause of the American declaration of war.

A second factor was the Zimmermann Telegram.  This was a telegram from Germany to Mexico.  In it, the Germans offered to give Mexico back the lands that it had lost to the US if Mexico would participate in the war.  Publication of the telegram (intercepted by the British) led to great anger among Americans.

A third factor, and one which some say was the major underlying cause of US entry, was the fact that the US had strong economic ties to England that were getting stronger as the war went on.  It had already traded more with England than with Germany, but the English blockade of Germany made the discrepancy much greater.  England also owed a great deal of money to the US.  Therefore, the US had an incentive, it is said, to ensure that England won the war.

These are three of the more important factors that caused the US to enter WWI.

There was no one Pearl Harbor-like moment or event that pushed the United States into World War I.  Instead, it was a combination of things that culminated in President Wilson asking for a declaration of war in early April of 1917.

One of the United States’ main issues in this war was the desire to trade with Europe.  Trade with Britain, in particular, was very important to the US economy.  For this reason, it had been opposed to Germany’s policy of unlimited submarine warfare.  Germany had backed off this policy to avoid antagonizing the US, but at the end of January 1917, it decided it had to resume that policy.  This upset the US.

The next event was the discovery of what is called the Zimmermann Telegram.  This was a message from Germany to Mexico requesting Mexican help in the war.  The Germans wanted Mexico to enter the war on their side to prevent the US from getting fully involved in Europe.  They promised to give Mexico back much of the territory it had lost in the Mexican-American War if it participated in WWI and if the Central Powers won.  This, not surprisingly, angered Americans greatly.

But even this did not actually cause the US to enter the war.  During March of 1917, three American merchant ships were sunk by German submarines.  These sinkings ended up as the last straw and Wilson asked for a declaration of war.

There are at least two different ways to look at American entry into this war. 

First, we can say that it came about for idealistic reasons.  President Wilson said that it was a war “to make the world safe for democracy.”  A major influence on the US decision to enter the war was the fact that Germany was violating international law through its use of unlimited submarine warfare.  The US wanted to make sure that international law and the rights of neutral countries were respected.  From this point of view, the US entered the war for good reasons. 

Second, we can say that it came about for much less idealistic reasons.  We can point out that England was also violating international law by blockading Germany.  From this point of view, the real reason that we entered the war was because it was in our economic interests to do so.  First of all, we had much more trade with England than with Germany, particularly after the war started and England blockaded Germany.  Second, the British owed us a great deal of money by the time 1917 came around and we wanted to make sure they won the war so they could pay us back.  This makes our entry into the war seem much less principled.