Here are the key reasons why France and Britain joined World War I:
1. Violation of Belgian Neutrality: The immediate trigger for France and Britain's entry into the war was Germany's invasion of Belgium. Belgium was a neutral country with whom both France and Britain had treaties guaranteeing its independence and neutrality. When Germany violated Belgium's neutrality, France and Britain felt obligated to come to Belgium's aid and uphold their treaty commitments.
2. System of Alliances: France and Britain were part of a complex network of alliances and ententes that obliged them to support each other in case of war. France had a formal alliance with Russia, and Britain had an informal agreement with France, known as the Entente Cordiale. When Russia mobilized its military in response to Austria-Hungary's actions against Serbia, Germany saw this as a threat and issued an ultimatum to Russia. France, as Russia's ally, began mobilizing its troops, prompting Germany to declare war on France and subsequently invade Belgium. Britain, bound by its agreement with France, joined the conflict in support of its ally.
3. Balance of Power: Both France and Britain were concerned about Germany's growing power and influence in Europe. Germany's rapid industrialization and military buildup since the late 19th century had raised fears among its neighbors. France, in particular, had lost the Alsace-Lorraine region to Germany in the Franco-Prussian War of 1870 and sought to regain it. Britain, on the other hand, was worried about Germany's naval ambitions and its challenge to British maritime dominance. By joining the war, France and Britain aimed to contain Germany's expansionism and maintain the existing balance of power in Europe.
4. Public Opinion: Public sentiment in both France and Britain played a role in their decision to enter the war. In France, there was a strong sense of national honor and a desire to avenge the defeat of 1870. In Britain, public opinion was initially divided, but as news of German atrocities in Belgium emerged, there was a growing sense of outrage and a willingness to support military action against Germany.
These factors combined to lead France and Britain to join World War I. The violation of Belgian neutrality, treaty obligations, concerns about Germany's power, and public support for action all contributed to their involvement in the conflict.