The war in Mockingjay is an extension of the battles in the Hunger Games arena described in The Hunger Games and Catching Fire. Before, the scale of war was much smaller, but just as lethal. The two tributes from each district were symbolic of their districts as a whole, creating a small-scale war in which each district fought the others, and each district also fought itself. Now the Hunger Games might be in the past, but the true battle is on. The Games arena has been replaced with the Capitol itself, and Katniss and her team have to employ the same kinds of strategies they relied on during the Games. More lives are at stake, though, and they have even more to lose this time.
For more on warfare in Mockingjay, check out "Why Should I Care?"