What was the circumstance that pulled most of Europe into war?

Most of Europe was pulled into war due to the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914. This event triggered a complex web of alliances and treaties, leading Austria-Hungary to declare war on Serbia. Russia mobilized to support Serbia, prompting Germany to declare war on Russia and subsequently France. The situation escalated further with Germany's invasion of Belgium, bringing Britain into the conflict, thus igniting World War I.