I am teaching a course in Defense Policy this semester. My background is in domestic issues on the social and financial side. So, encountering this literature is very interesting.
1. Much of the discussion is ideological--whether it be about a justification for a specific service, weapon, policy, etc. Lots of evidence is offered, usually cherry-picked, but almost always the end point is given at the beginning.
2. Uncertainty plays a very large role. At the same time, much can be done to tame uncertainty with better intelligence, systematic analysis (and a sense of where arguments or evidence are most vulnerable), and some of the ideological or strategic or doctrinal ideas from #1. Whether that taming is good or diversionary is the big issue.
3. More than in most other policy areas, the "other" gets a vote. Hence, your enemy may make moves that are not only surprising, but are meant to be surprising, or are meant to be disinformation or distractions. Moreover, your other may be using a very different playbook than you are.
Related to #2 and #3, can you have some sort of option to revise or reverse your commitments, without having to pay too much for this option
4. Almost all talks of crisis or gaps or ... are scare tactics. There really are scary scenarios, or at least ones that require consideration, but usually as in #1 most such scenarios are in fact meant to push you in a direction rather than warn you of a problem.
5. Labor (soldiers!) is now expensive and represents a massive investment in training. Capital (equipment and technology) is less flexible than one would like, unit costs are in general high, yet the US needs a technological edge as much for reassurance as for fighting.
6. Getting the prices right is one problem, the other is getting the quantities right. By the latter I mean the bureaucratic organization, including jointness, unity of command, and internal conflicts that use up resources. Moreover, the combatants in this case are supported by external players (Congress, lobbying by firms and by services) who have a big say. The President may be Commander-in-Chief, but even five-star Eisenhower had to work very hard to have his commands implemented. (We know that in bureaucracies, there are many ways to slow down the effects of the higher ups.)
In general, each of the services has a doctrinal rationale for its primacy and necessity. But what is needed is a sense of the tasks and contingencies, and a doctrine that allows the services to work together to achieve the goals.
7. Fashions and Fears: Will our service or way of warfighting become obsolete. Should we have light footprint, COIN, regular land/air war configurations, what is the role of the Navy?, etc. Much of this is related to #6.
8. In times of budgetary stringency and downsizing (usually after wars), the problem is not the decreased budget. The problem is always the rigidity of your budgetary commitments due to long term plans and sunk assets, external and internal constituencies, and the ability of opponents to construct perhaps-credible accounts of what might happen.
9. Right now DoD produces Veterans, while VA deals with the products. There should be some way of internalizing the costing, to incentivize DoD policy. And VA should not be in the disability and entitlement business. The VA should be in the reinvestment business, transforming some of the population into productive civilians. Probably, there needs to be major research efforts of the next two decades, to do a better job in healing, reinvesting, and transformation. Too many private organization (profit, and even nonprofit) feed at the teats of the VA, often unscrupulously. (For profit higher education, nonprofits that do little except collect contributions,...)
10. You will always have scandalous behavior in such a large system as Defense. But each such scandal may suggest ways of diminishing such, without at the same time making for bureaucratic red tape. Hence, contracting and logistics, sexual harassment and violence, and narcotics, demand responses that actually make things better. Command structures are bound to be altered, for it is their leadership failure that is most troubling.
11. I believe that armed services have benefitted from the last 60+ years of less-discrimination or nondiscrimination, with stronger troops and officers. As more women move up in the services, improvements will become apparent. (As they have become apparent in the Congress.) What will become clear is that many issues and problems that were consequences of selective bias will be attended to.