If you are talking about what was the trigger for World War I, it was one of the following:
<u>Extended Alliances:</u> There were two key alliances, one called the Central Power, and the other called the Allied Power. The Central Power (containing Germany, Italy (which later switched sides), Bulgaria, the Ottoman Empire, and colonies) were called Central, because they were located near the 'center of the world', and mostly in central Europe. On the other hand, was the Allied Powers, which was a mixture of countries, from Serbia, to Great Britian and her colonies & protecterates, France, Russia (before U.S.S.R, in the time of the Czars), Japan (& her colonies), the US (after the sinking of the Lusitania and the Zimermann Telegram), and other nations.
<u>Assassination of Archduke Ferdinand and Sophia:</u> Before the start of World War I, we must know what the cause of the war was about. In the months and years before the war, Austrio-Hungary tried to annex Serbia under their rule, which did not make the Serbs and Russians happy, because:
1) Serbs wanted to be independent, and wanted to keep Serbia
2) Because of the large Slavic population in Serbia, Russia wanted to keep Serbia for their own interests (as they are Slavic too).
This led to the assassination of Archduke Ferdinand and Sophia by Princip, a Serb. Even though Serbia tried to pay reparation to Austrio-Hungary, Hungary took the chance to try to take over Serbia, and declared war, triggering the extended alliances into fighting each other.
<u>US Involvement:</u>
Because of her mixed population, many in the US believed that staying out of the war was the best choice. However, the US was pro-Allied, and supplied them with war material. A number of reasons led the US to join the Allied side militarily:
1) <u>Sinking of Lusitania:</u> The British liner, <em>Lusitania</em>, containing 128 Americans, was sunk by a German submarine. Even though the liner was carrying contraband for a steam liner, it led to public outrage in the US, and was the first step to the US being involved in World War I.
2) <u>Zimmermann Telegram:</u> The Zimmermann Telegram, sent by the German minister Zimmermann, offered to, if Mexico would assist them, to help them reclaim land previously lost in the Texas Independence and the Mexican-American War. This proved to be the last straw for the US, which joined World War I on the Allies side, and was able to turn the war around for the Allies favor.
hope this helps
