After finding and exploiting a gap in the German lines at the Marne, the British and French armies pursued their adversaries as quickly as the armies could march.  The Germans fell back to close the gap and keep the Entente forces from being able to either turn into the flank of their troops or get loose in their rear areas.

The Germans stopped retreating when they reached the Aisne River.  There, the ground rises into steep cliffs at the Chemin des Dames, with good fields of observation and fire.  The Germans took advantage of this terrain to create a belt of trenches and strongpoints.  This is where they would stand and fight to keep what they had gained in Belgium and France.

The British and French threw themselves at these emplacements, but no matter how motivated and courageous the troops were, they could not overcome concentrated machine guns and artillery on higher ground and behind entrenched infantry.  The lesson the Germans learned at Liege was taught to the Entente forces, and they were unable to dislodge the Germans.

After losing thousands of men with nothing to show for it, the Entente dug in opposite the Germans.  This began the hardening of the Western Front into a continuous line of trenches that would eventually run from the Belgian seacoast to the Swiss border.  In a battle of movement, there are times when the combatants cannot get at each other, and casualty rates will drop for a time. In trench warfare, the combatants are always within range of one another, and no such reprieve is possible.

In order to not repeat the mistake of frontal attacks against entrenched enemy troops, both the Germans and the Entente powers tried fruitlessly to turn the flank of the other.  In this “Race to the Sea”, a German move would be thwarted by a British or French maneuver, or vice verse, with both armies slipping continuously to the northeast until they finally ran out of land.  Once their sleeves were brushing the sea, the war of movement ended, and the stalemate of trench warfare, and the daily grinding down of armies, began.

The resulting line of siege works kept the belligerents at each others throats for years, and left no alternative but frontal assault for commanders.  This was a style of war that neither side was trained or equipped for, and the on-the-job training that both troops and commanders went through took a horrendous toll in blood.