No it is not the 400 mile line of trenches. The term Race to the Sea is the name of all attempts to sourround an enemy by French Army or the German one.
I'm sure you remember the Battle of Marne. As a result the German advance in France was stopped. The AF and BEF forced the German Army to retreat east. They moved back to the Asine river line and made a line of trenches. French Army with the BEF together attacked but the German defence was very strong. So general Joffre ordered to stop the attack. The AF and BEF digged out their own trenches along the German's line.. Then both Armies ( I mean the French one + BEF and the German one ) tried to outflank the enemy alternately , going each time more and more north toward the sea. Each attack ended with stopping the enemy advance and digging trenches what was making the line of trenches longer going more and more to the north.
Well the answer to your question when the race was started can be ... the Battles of Marne and Asine were the prelude to the race. The first battle that started that was the Battle of Picardy, 22–26 September 1914.
I would say you should have a look at the Race to the Sea - Wikipedia
I'm sure you remember the Battle of Marne. As a result the German advance in France was stopped. The AF and BEF forced the German Army to retreat east. They moved back to the Asine river line and made a line of trenches. French Army with the BEF together attacked but the German defence was very strong. So general Joffre ordered to stop the attack. The AF and BEF digged out their own trenches along the German's line.. Then both Armies ( I mean the French one + BEF and the German one ) tried to outflank the enemy alternately , going each time more and more north toward the sea. Each attack ended with stopping the enemy advance and digging trenches what was making the line of trenches longer going more and more to the north.
Well the answer to your question when the race was started can be ... the Battles of Marne and Asine were the prelude to the race. The first battle that started that was the Battle of Picardy, 22–26 September 1914.
I would say you should have a look at the Race to the Sea - Wikipedia