Tactics
Blitzkrieg (Germany)
The Blitzkrieg, or "Lightning War," was a tactic used by the German military that was designed to destroy the opponents with quick and high powered forces. The first wave of Blitzkrieg was fast moving and large planes and tanks that would go into and destroy the major military that they are attacking. The second wave of Blitzkrieg was fast moving, smaller tanks and infantry that would finish off the left over military members still alive.
Submarines
Submarines was a war tactic that many countries used in an attempt to sneak up on the opponent without them knowing. Naval fleets would sometimes use submarine tactics to approach a location and surprise attack and harm as many members of their military as possible and then bring in backup military to finish off whoever is left. Submarines were a dangerous form of war, because they are hidden within the waters.
Trench Warfare
Trench warfare was often used by the Soviet Union's Red Army. The trench warfare was a type of land tactic that the militaries used to stay protected from the opponents. The military would build large trenches in the ground and hide in them during a time of war, when the opponent would approach the trenches the military could then fire at them while staying sheltered.