Do to some internet issues, today's post will be brief. Sorry.
Barbed wire is used an a lot of modern warfare. Originally, like most things, it was not invented for war.
In 1873, Joseph Glidden invented it to keep in cattle. At one point in time, cattle were allowed to roam free, without fences or people watching them. Because the cattle went were they wanted, they grazed on anyone's property and drank from any water source. This was fine so long as there were only ranches in the area and there had not been a recent drought.
Then, when droughts set in, ranchers began to need to keep their cattle on their own land, near the water source they owned. Now they were putting up fences to keep other ranchers' cattle out.
The size of cattle made it so that often they would just tear through the fences or knock them over. Barbed wire solved this problem in that the cattle would be hurt if they came too close, so they stayed away.
During WWII, the military realized that barbed wire could be used in trench warfare. When laid out in No Man's Land between the trenches, it made any charge across the field so slow that the enemy could be easily picked off before they got through. Barbed wire coupled with the machine gun completely changed warfare as it would now be complete suicide to have armies line up and charge each other as they did in previous wars.