Several critical events influenced the United States to join World War I. The sinking of the Sussex, a French liner carrying mostly civilians and non-war materials, led to the Sussex Pledge, where Germany agreed to end unrestricted submarine warfare to avoid provoking the US further.
This note was a note from Germany intended for Mexico, which was intercepted by the Allies. This note basically said Mexico should declare war on the US over the land that Mexico had lost in the Spanish-American war, and that if they did, Germany would pay for all of the damage caused by the war. Mexico obviously did not accept this offer.
Russian Revolution
Russia, at the start of the war, was a major power country for the Allied powers. But in the middle of the war, the citizens began to rebel against the government, and the Russian government ultimately decided that it would be best if the withdrew from the war and fixed the problems within their country, rather than having the war to worry about. While this may have been the best decision for Russia, this left the Allied powers high and dry, and looking for another major power to be on their side. This is where the US comes in.
Sinking of the City of Memphis,
The Illinois, The Vigilancia, and
The Arabic
After the Battle of Jutland, Germany realized that they would never beat the Allied Powers by overpowering them, it just wasn't possible. They decided that they would end unrestricted u-boat warfare, and start to sink US ships headed to Britain. Germany decided that they would hurt them in their pocketbooks, by sinking the ships carrying goods for them to sell in their country.
Sinking of the Sussex
After sinking the Lusitania, the Germans continued to sink random ships. They thought that by sinking ships, they would really hurt the US. The Sussex was a French Liner that was carrying mostly civilians and a few materials that weren't war related. After sinking this ship, the US encouraged Germany to stop sinking random ships. This began the Sussex Pledge. This pledge ended unrestricted u-boat warfare.
Sinking of the Lusitania
This began the idea of unrestricted u-boat warfare. Traditionally, ships would follow cruiser rules. These rules said that the countries shouldn't sink ships carrying civilians, and if they did, they should allow the civilians time to exit the ship first. Because these new u-boats couldn't carry any more passengers, and were designed to be stealthy under the water, these rules were obviously not being followed. And because the Lusitania had around 126 American passengers on it, the Germans angered the US.
Causes of WW1
Alliances
Alliances caused the first world war because countries were so loyal to others. Many countries that weren't even involved in the initial conflict got pulled in because the countries they were aligned with wanted their help.
Militarism
Militarism caused the first world war because many countries built up their militaries during World War 1 in order to defeat other countries. The Germans initially encouraged Austria-Hungary to declare war on Serbia because they reasoned that their military was as strong as its ever been, and that this war was bound to happen eventually, so why give other countries time to mobilize and prepare for war.
Nationalism
Nationalism means identification with one's own nation and support for its interests, especially at the detriment or exclusion of the interests of other nations. This type of thinking caused many people to believe their country was the best or the strongest. It lead people to believe that they would win the war because of their superiority, so may countries just blindly entered the war since their allies were in it, without really thinking about the consequences.
Imperialism
Imperialism, you could say, was the main spark of the war. The definition of imperialism is, the quest for colonial empires. Both Serbia and Austria-Hungary were trying to annex a country called Bosnia Hersh Covina. Serbia wanted to annex Bosnia because they were both Slovac-speaking countries, but Austria-Hungary beat Serbia to it and annexed Bosnia first. This angered Serbia because they wanted Bosnia. So, when the Archduke of Austria-Hungary was going through a main city of Bosnia, a Serbian nationalist group decided to assassinate him and his wife. This obviously angered Austria-Hungary and they gave Serbia 48 hours to turn over the men. It ended up being a month, but then, Austria-Hungary declared war on Serbia. So you could argue that the main cause of the spark of World War 1 was Imperialism.
