This poll of defense insiders indicates that the legacy of Robert Gates is Iraq, more or less. Steve Metz and I disagree. Metz tweeted: "repairing the relationship between the SECDEF and senior military leaders." Mine was: "rescuing OSD and SecDef's rep after Rummy's mess." Perhaps Iraq kind of addresses those to a certain degree, but I definitely think that Gates made a huge difference, and perhaps we appreciate him all the more because his predecessor was the worst SecDef in US history.
The survey then identifies what should be Gates's successor's priorities--getting out of Afghanistan and cutting the budget of the military. Both are important, but are subsumed by this: figuring out what will American military power be used for in the next generation and preparing the military for that. How? By reducing current commitments (Iraq, Afghanistan, Europe) and shifting spending so that the declining budgets will be well spent.
Of course, politicians will seek to send troops and ships and planes where the military might not want and where the SecDef did not plan. But that is why plans include contingencies. The question that should be asked of each line of the defense budget is: how does this commitment of scarce dollars affect the choices down the road? If a certain plane is so expensive, that it reduces the flexibility of future commanders, that would be a good reason to downsize or cancel a program. If a certain system increases flexibility and the ability to exert influence, that would be worth spending money on. We face an era of tradeoffs, and the SecDef's job will be managing those and recommending to the President the best ways to manage (not eliminate) risk.