While the single shot rifle remained the mainstay of the infantry up until the early days of WWI, machine guns had been in development since the final days of the American Civil War. In the later years of WWI, machine guns were used on both sides with devastating impact and by WWII, along with assault rifles, they had become the mainstay of the infantry. But these weapons also found their way into the hands of criminals, insurgent groups and failed states because they required little military training to be effective.