During the majority of WWI, the United States wanted to remain isolated in what many considered to be a European conflict. President Woodrow Wilson himself advocated isolationism until a series of conflicts drew the US into war against Germany.
First, even though Britain was at war, the US remained an active trading partner. The US continued to send merchant ships to and from Britain. In 1915, Germany announced unrestricted warfare against all ships around Britain, including the American merchant ships. Inevitably, the Germans sank an American ship, angering the US public and President Wilson. However, this wasn't enough to get us to declare war on Germany.
Later in 1915, the Germans sank the Lusitania, a passenger vessel traveling around Britain. The Germans claimed that there were munitions on board, so the attack was justified. While it turns out that the Germans were correct in this assumption, it did not matter to the US- 128 Americans had died on the Lusitania. This was a major outrage for American citizens, and public opinion started to shift away from isolation and neutrality.
In 1917, a German telegraph sent from Germany to the German US ambassador was intercepted. This message said that in the event of war against Germany, Mexico should be asked to be a German ally, as a way of damaging the US.
Ultimately, Germany's refusal to stop attacking allied ships, policy of unrestricted submarine warfare, efforts to gain allies against the US, and US public opinion encouraged Congress to declare war against Germany in April 1917.
The US was unable to remain neutral because of flagrant attacks on their ships, relationship with European allied powers, and a strong anti-isolationist public sentiment that developed between 1914 and 1917
The major reason for this was the fact that the United States wanted to be able to continue trading with the countries of Europe. This was an important thing for the US economy since losing European trade would have meant the loss of a great deal of money.
But Germany and England did not want the US to trade with their enemies. The British blockaded Germany using surface ships. The Germans blockaded England using submarines. The German blockade led to the sinking of various ships carrying American citizens. As this grew more common, the US became more and more inclined to join the war on the side of the Allies.
If the US had not felt the need to trade with Europe, it would probably have been able to remain neutral. However, since it wanted to keep trading, it inevitably got pulled into the war because of the fact that the Germans needed to prevent England from getting supplies by sea.
The United States was unable to remain neutral in World War I. There are several reasons for this. One reason was we were more sympathetic toward the British than toward the Germans. Our language and system of government are very similar. We did a lot of trade and business with Great Britain. Thus, we felt closer to Great Britain than we did to Germany.
Another factor was the Germans interfered with our trade. The Germans used the submarine to attack our ships without warning. This was illegal since we were a neutral country. We had the right to trade with any country since we were neutral. No country could interfere with that trade. If a country at war wanted to search our ships, they could do that. If we were carrying ammunition and war supplies to a country at war, then the country searching our ship could seize the ship and its cargo or remove the crew and sink the ship. Germany didn’t do this. They sunk our ships without warning. When Germany agreed to stop doing this, things got better for a period of time. However, in 1917, Germany began to again sink our ships without warning. This brought us into the war.
We also were displeased that Germany tried to get Mexico to attack us. When this news became known, people were angry at Germany. This added to the pressure to go to war against Germany.
While the interference with our trade was a major reason for going to war against Germany, other factors also played a role in this decision.