Austria attacks Serbia in response to Archduke Ferdinand’s assassination. Russia, allied with Serbia, declares war on Austria. Germany, ascendant, could not let Austria fall to Russia. Germany declares war on Russia. France, allied with Russia, declares war on Germany. Great Britain could not afford Germany becoming the dominant power on the continent and, allied with France, declares war on Germany. After three months stalemate and trench warfare are the norm on the Western Front with no-one being able to achieve a breakthrough and no-one seeing any way out. It wasn’t until we poured in one half million doughboys in late 1917 that the tide was finally decisively turned.
Britain’s mistake was not doing a Brexit in 1914. Why did she care so much for the Continent, when she had her Empire. Where did this sudden love for France come from anyway, after they had been enemies for centuries.