After gaining the backing of Ford the Black Legion drove the Jews out of the Detroit area (much to Ford's dismay). There were beatings, the Temples were closed, property and money was seized (which helped fund the Black Legion as much as the money from Ford did). Some fled to Canada, others to Chicago. From these refugees and the Jewish population of Chicago the Jewish Defense Force was created. It was the creation of a group of respectable Jewish leaders, and some very un-respectable Jewish gangsters. Detroit had been the home of the Purple Gang, a group of Jewish American gangsters who were involved in bootlegging, gambling, the protection racket, and other types of organized crime. When the Purple Gang showed up in Chicago Capone's outfit (which had previously gotten along with the Purple Gang) grew concerned about them muscling in on their territory. To skim off the younger hotheads from the Purple Gang, Capone funded and supplied arms to the newly formed JDF. Suddenly, guys who had been looked down on as hoodlums found themselves cheered as champions against anti-Semitism.
Unlike many other newly formed "armies" (such as the Unionmen or the Negro Defense Force) the JDF is well equipped with automatic weapons. Every "squad" has at least two Tommyguns and frequently a BAR. They were the first of the irregular "armies" in the region to carry grenades, and their grenades were regular issue ones (the other irregular armies of the region had to make do with homemade grenades and bombs that often didn't work right). They lacked much in the way of heavy weapons, but were well supplied with cars and trucks. This mobility caused them to be regularly used as a fire-brigade, to reinforce threatened positions. With so many of the members having suffered at the hands of the Black Legion they were very dedicated, if not all that well trained or competent. No actual uniforms at first, just regular clothes and armbands (the former members of the Purple Gang going in for fine suits when work clothes would have made more sense).