Why did france and great Britain join World War 1?

France and Great Britain joined World War I primarily due to their alliances and commitments to defend each other against aggression. France was concerned about German expansion and sought to reclaim territories lost in the Franco-Prussian War, while Britain was committed to supporting Belgium's neutrality after Germany invaded it. The complex web of alliances, including the Triple Entente between France, Britain, and Russia, ultimately compelled both nations to enter the conflict when war broke out in 1914.