The U.S. military has over 1,000 military bases, distributed over 20 countries, containing at least 290,605 buildings (warning: PDF). Each of those hook into the military networks that remain targets, and prominent ones at that. There are countless stories about successful breaches of military infrastructure to gain information, at least according to what is publicly available. There are also those who target them as part of political activism. Overall, their role as one of the largest targets in the world is a known problem.
The U.S. Department of Defense announced in September that they intended to create a Joint Information Environment. This would involve integrating the various networks that they control into a single controlled design. In doing so, it would dramatically reduce the threat surface that the largest network in the world faces.
Currently, the Department of Defense has broad guidelines (warning: PDF) to allow for communication between branches. This has led to the development of improved systems that are foundational to the next stage of integration. For instance, C2 Central allows the sharing of information between the hundreds of networks across the branches. It also demonstrates the scale of the problem. The difficulties faced with the BACN project have also shown the layers of sensors, networks, and even basic lack of networking that the pieces of the military infrastructure contain.
The larger the environment, the harder it is to create universal policy. Powerful interests come into play at every juncture. Some of it is the desire to keep fiefdoms functioning. Some of it is disagreement over the choices made to remove the systems used in certain circumstances. There are as many reasons why not to integrate as there are players involved.
Regardless, reducing technologies and networks in play reduces the attack surface. It also increases the need for confidence in those technologies that are chosen in this reduced environment. The military has faced problems before in making poor choices on this matter. The project also faces potential conflicts with its BYOD strategy. The process involved would most likely take years, most likely at least a decade. It will involve process design, technology replacement, and retraining of every single person who interacts with the redesign. Even with the best of intentions, this will not be an easy task.