How did the world war 1 end up involving so many nations?

The first World War began as a local war between Austria-Hungary and Serbia in 1914. It grew into a war involving 32 countries. The Allies included Britain, France, Russia, Italy and the United States. These countries fought against the Central Powers which included Germany, Austria-Hungary, Ottoman Empire and Bulgaria. Archduke Ferdinand, of Austria-Hungary, was assassinated by a Serb on June 28, 1914. This was the formal start of the war, although the countries had problems with each other before that. The countries of Britain, France and Germany had all grown strong and had large armies. They all wanted to sell their products to foreign countries. Several times Britain and France had been in strong arguments with Germany over markets in Africa. Europe was divided into two groups who were hostile to each other. After the assassination, Austria declared war on Serbia. A few days later, Germany declared war on Russia, then on France. As the war progressed, other countries took sides and declared war. Finally the United States entered the war against Germany on April 6, 1917.