Each of these pictures shows an advance in technology in WWI.  The tank at the bottom symbolized the ability to combine speed, armor, and firepower on the battlefield and enabled one side to cross no-mans land.  The trench gave the infantry more cover which helped them to survive long and to resist more heavily.  Airplanes at the beginning of the war were used as observatory aircraft.  Then they were equipped with machine guns and were used as combat machines.