On the western front, the major military strategy that led to four long years of war was

During World War I, the opposing sides dug into trenches to hold their ground.  If they would try to mount an attack against the other side, venturing into "no man's land" generally meant getting mowed down by machine gun fire.   In addition to trenches themselves, the soldiers would stack sand bags along the top of the trench to form a protective wall.  The sand bag walls were effective against rifle fire, but not terribly effective against artillery fire from the opposing side.  In addition to bullets and mortar fire, trench warfare also came to involve the use of chemical weapons like mustard gas.

Trench warfare in World War I was miserable and gruesome.  Check out All Quiet on the Western Front by Erich Maria Remarque (1929) for first-hand descriptions of the misery of the trench warfare.

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after the roman empire, the catholic church began to unify by taking on many duties of a government.