Austria-Hungary gave an ultimatum for Serbia, on July 23, 1914 connected to the killing of Archduke Franz Ferdinand, and declared war on July 28. Do to various treaties in place, Russia entered the war the following day, July 29 on the side of Serbia and started to mobilize its arm.
Germany was determined to support Austria-Hungary in anything they did, because they were afraid of losing their last remaining ally. That could either mean by angering them so they withdrew their support, or by allowing Austria-Hungary to decline into international irrelevance, in which case their friendship would be of no strategic value. Germany strongly encouraged Austria to declare war on Serbia, so that Austria could prove it was still a Great Power able to enforce its will on lesser nations.
When Russia declared on Russia, France came to her aid both, because of their alliance and France was protecting its ecominc interest. It needed Russia to survive the war
Britain entered World War I in 1914 to honor a treaty that promised to defend Belgium and to fulfill an alliance with France and Russia.
Russia got into the war because of their alliance with Siberia. There was a conflict when the prince of Austria was assassinated by a radical group called the Black Hand. Austria blamed Siberia saying that Siberia's people did it. Austria sent an ultimatum to Siberia saying if Siberia didn't meet their demands, and confess to the assassination, Austria would declare war on them. When Siberia refused saying they didn't have anything to do with it Austria declared war on Siberia, and Russia mobilized her army to protect the people of Siberia, and to keep true to their word to do.
A secret treaty was concluded between the Ottoman Empire and the German Empire on August 2, 1914. The Ottoman Empire was to enter the war on the side of the Central Powers one day after the German Empire declared war on Russia.
Italy declared itself neutral in the conflict, despite its membership in the Triple Alliance alongside Germany and Austria-Hungary since 1882. The decision to join the fray on the side of the Allies was based largely on the assurances Italy received in the Treaty of London, signed in April 1915. By its terms, Italy would receive the fulfillment of its national dream: control over territory on its border with Austria-Hungary stretching from Trentino through the South Tyrol to Trieste.
Bulgaria wanted to regain territory lost in previous war. Germany and other Central Powers offer big territory if Bulgaria entered the war.
United States entered World War 1 because of the unrestricted attacks German submarines were making on American ships and ships containing American passengers. Additionally, Americans were enraged at a telegram British intelligence intercepted in which Germany sought an alliance with Mexico.