The shorthand version is that the French had a strong line of fortifications between itself and Germany, the Maginot Line. The Schlieffen plan involved going round these defences by way of a sweep down through Holland and Belgium and taking out Paris before the French could mobilise. Everything hinged on speed. Britain may not have got involved in the war had it only been between France and Germany, but it was treaty bound to defend Belgian neutrality. So when Germany entered Belgium, Britain declared war and sent the BEF to France. Although the German plan worked well initially, and they got within sight of the channel ports, which would have cut the BEF retreat, the British dug in at Le Cateau and the French at the battle of the Marne, and the German advance ground to a halt. Once the momentum was lost, both sides dug in and settled for four years of trench warfare.