WW1: A question about tactics in World War One ?

At first, their Germans under their Schliffen Plan moved fairly rapidly through Belgium and Northern France. They were stopped at the Marne, and trench warfare set in. Machine guns, barbed wire, and trenches were all designed for defensive warfare, and thats what WWI became, a giant stalemate, atleast on the Western front. thousands would die for yards of ground. Later in the war, Tanks were used by the British and French to break the stalemate, insuccessfully at first, but as more were deployed at once, the Germans became overwhelmed. So it started went from attack at the utmost, to trench warfare, to early armored warfare.