While Tanks from Gale Force Nine puts the emphasis on the machines of war as they roll across the battlefields, blasting at each-other, one would do well to remember that without the crews, those vehicles would just be masses of metal and plastic sitting motionless on the battlefield. And those crews all came from different countries with distinct philosophies towards war. In these articles from Gale Force Nine, we get a look at some of the different Doctrine cards, as well as a look at the special card in the Mechanics kit.
Doctrine cards give a bit of national flavor to each tank. If you took a British crew and tossed them in a Russian tank, the gear might change, but the way war is made will be the same. It's that sort of flavor that these cards look to put into the game.
Meanwhile, the Mechanics kit is a set of clippers, a file, and glue for putting together your kits. But coupled with all that is a new crew card. So not only can you keep your tanks in good working order physically, but also in the game, itself.
Doctrine Cards Source
Mechanics Kit Source