World War I
The new machine guns, artillery, trenches, and mines made it very difficult to attack. The generals had fought many wars without them and so they ordered their armies to attack in the old style of marching in rows, which allowed the enemy to shoot them down easily. At the 1916 Battle of the Somme, 60,000 British men died in a single day, one of the bloodiest days in the history of the British army. Late in the war, the British and French invented tanks and used them to attack entrenched Germans, but they could not make enough of them to make a big difference.  The German Empire responded by making their own tanks but they had even fewer.  The Germans also invented special Sturmabteilung tactics to infiltrate enemy positions.  This was also too little, too late.