A new technology at the beginning of WWI that made it difficult or almost impossible to cross defended ground due to tactics such as trench warfare; it would later be used on tanks and airplanes.  The machineguns could fire many bullets at one time and they caused more casualties at a faster rate. 

Airplanes were used early in the war as spotters and reconnaissance. The later were used to bomb ground targets and in aerial combat.

Chlorine gas and mustard gas was used to create a smoke screen to hide attacking soldiers. It forced the evacuation of enemy trenches and caused almost certain death to those who inhaled or came in direct contact with the gas.

A type of combat in which opposing troops fight from trenches facing each other. Fighting in the trenches caused stalemate as no one side could advance on the other and gain any ground.