What led France and Britain to declare war on Germany to begin world war 1?

France and Britain declared war on Germany primarily due to Germany's invasion of Belgium on August 4, 1914, as part of the Schlieffen Plan to quickly defeat France. Britain had a longstanding commitment to protect Belgian neutrality, established by the Treaty of London in 1839. Additionally, France was allied with Russia, which was mobilizing against Germany and Austria-Hungary. These alliances and obligations escalated the conflict, leading to a broader war.